Photoresponsive Polymers

C H A P T E R

4

Photoresponsive Polymers X. Xiong, Aránzazu del Campo, J. Cui INM—Leibniz Institute for New Materials, Saarbrücken, Germany

4.1 INTRODUCTION Photoresponsive polymers undergo a change in their properties in r­esponse to a light stimulus (Irie and Ikeda, 1997; Shibaev et  al., 2003; Zhao, 2007; Krauss et  al., 2010). Different molecular properties can be light-­regulated, including conformation (Shinkai et  al., 1982; Nor et  al., 2007; Lai and Hong, 2010; Gupta et  al., 2008; Ruchmann et  al., 2011; Everlof and Jaycox, 2000; Irie, 1993), polarity (Zakhidov and Yoshino, 1995; Wu et al., 1999; Hidayat et al., 2002; Sajti et al., 2002; Pandey et al., 2012; Wang et al., 2012), amphiphilicity (Chen et al., 2011; He and Zhao, 2011; Chen et al., 2012a; Han et al., 2012), charge (De et al., 2010; Fries et al., 2010; Byrne et al., 2011), optical chirality (Mayer and Zentel, 1998; Zhang et al., 2011a), conjugation (Kim et al., 2005; Uchida et al., 2005), etc. The light-­induced molecular change is reflected in a macroscopic change of material ­properties such as shape (i.e., contraction or bending), wettability, solubility, optical properties, conductivity, adhesion, etc. Light-control possesses intrinsic advantages compared to temperature, pH, electric, and magnetic stimuli: (i) noncontact and remote control, (ii) can be easily dosed to tune the strength of the response, and (iii) allows accurate temporal and positional resolution of the response. The functionality and, ultimately, the application potential of such a polymer are mainly determined by three parameters: (i) the magnitude of the property change after light triggering, (ii) the rate at which this change occurs, and (iii) the reversibility of the process. In general, an ideal responsive polymer is one that exhibits instantaneous and drastic property variation upon light exposure. Depending on the application, a modulation of the response with the light intensity or a reversible property change may also be advantageous. To obtain photoresponsive polymers, a photoresponsive functional group (chromophore) needs to be incorporated into the polymer chain.

Smart Polymers and Their Applications https://doi.org/10.1016/B978-0-08-102416-4.00004-1

87

© 2019 Elsevier Ltd. All rights reserved.

88

4.  Photoresponsive Polymers

Depending on the type of chromophore used, the response can be reversible or irreversible. Reversible systems can alternate material properties in two photostationary states and are used as switches. Reversibility is important in many applications such as information store (Feringa et al., 2000) or artificial muscles (Ikeda et  al., 2007) and actuators. Irreversible chromophores are mainly photocleavable, which could be applied to photoinduced micropatterns, photodegradable materials, and controlled drug delivery. The advantage of irreversible chromophores is the possibility of 100% photoconversion because no equilibrium between two states is involved. This leads to the effective release of drugs (Zhao, 2009) or to a drastic decrease of the molecular weight in degradation application (Pasparakis et al., 2012). Different aspects of photoresponsive polymers have been recently reviewed: Fustin and Gohy summarized reported work on photoresponsive block copolymers (Schumers et al., 2010a); Zhao and coworkers reported their light-induced self-assembly, copolymer micelles, and light-cleavable main-chain photoresponsive polymers (Zhao, 2009; Zhao, 2012; Gohy and Zhao, 2013; Yan et al., 2013); Yan et al. summed the photoresponsive polymeric micelles (Huang et al., 2014); Theato et al. reviewed the photosensitive polymers containing photoremovable groups (Zhao et al., 2012); Das and coworkers reported pseudorotaxane-based photoresponsive assemblies (Mandal et al., 2015); Tian et al. reviewed the functional host-guest photoresponsive system (Qu et  al., 2015); and Wang et  al. reviewed the amphiphilic azo polymers and their photoresponsive properties (Wang and Wang, 2013). In addition, several reviews about azobenzene-based polymer systems (Goulet-Hanssens and Barrett, 2013; Bushuyev et  al., 2017; Wei et  al., 2015; Yu and Ikeda, 2011; Ikeda et  al., 2007; Iqbal and Samiullah, 2013; Yu, 2014; Mukhopadhyay et al., 2014), light-triggered actuation (Priimagi et al., 2014; Bisoyi and Li, 2014; Bisoyi and Li, 2016), supramolecular systems (Lee and Flood, 2013; Jones et al., 2016; Draper and Adams, 2016), drug delivery (Cho et al., 2015; Rwei et al., 2015; Bansal and Zhang, 2014; Swaminathan et al., 2014; Linsley and Wu, 2017), and so on (Wondraczek et al., 2011; Zheng et al., 2013; Al-Malaika et al., 2010; Barrett et al., 2007; Ercole et al., 2010) have been recently reported. We also highlighted the functionality and application of photolabile polymer at surfaces (Cui et al., 2013a). In this review, we focus on representative examples of recently developed polymer systems incorporating p ­ hotosensitive groups excluding blends (Suzuki and Tanaka, 1990) and self-­assembled (Wang et  al., 2007; Willerich and Gröhn, 2010) photosensitive polymers, which have been recently reviewed elsewhere (Yu and Ikeda, 2011; Yagai and Kitamura, 2008). We introduce the main types of chromophores and photoresponsive polymers and their properties in Sections 4.2 and 4.3. Special attention is paid to supramolecular polymers as one of most relevant developments in the last several years. The main applications for these



4.2  Chromophores and Their Light-Induced Molecular Response

98

systems are described in Section 4.4, including controlled drug delivery, patterned thin films of hydrogels and polymer brushes, photodegradable materials, and liquid crystal actuators. Finaly, we give our critical view of the field and its future development.

4.2  CHROMOPHORES AND THEIR LIGHT-INDUCED MOLECULAR RESPONSE Chromophores can be classified into two categories: reversible and irreversible. Reversible chromophores, often named molecular switches, undergo a reversible isomerization upon light excitation at a specific wavelength. The photochromic interconversion between isomeric forms allows switching the properties of the polymer material by irradiation at two different wavelengths. Fig. 4.1 presents some examples. Azobenzene

FIG. 4.1  Typical examples of reversible and irreversible photoresponsive groups.

90

4.  Photoresponsive Polymers

alternates between planar trans form and bent cis form via light-induced isomerization of the NN bond (Barrett et al., 2007). When coupled to a polymer chain, azobenzene has enabled switching of hydrophilicity (Zhao, 2012), chirality (Maxein and Zentel, 1995), optical properties (KozaneckaSzmigiel et al., 2011; Kravchenko et al., 2011; Alicante et al., 2012; SchabBalcerzak et  al., 2012), and coordinative interaction (Barille et  al., 2009; Royes et al., 2012) in polymer materials. Spiropyran changes from an unconjugated spiroheterocycle to a charged planar merocyanine (MC) form with extended conjugation (Paramonov et  al., 2011). The light-induced change of a neutral to a charged system has been applied to control wettability (Anastasiadis et al., 2008), vesicle dissociation (Lee et al., 2007), molecular recognition (Andersson et al., 2008), solubility of polymer chains (Szilagyi et al., 2007), and ion penetration (Nayak et al., 2006). Ultraviolet (UV) irradiation of a spirooxazine initiates an electrocyclic ring-opening reaction of a closed spiro form, which results in the formation of an open MC form with an extended conjugated system able to strongly absorb in the visible region (Nori and Chu, 1983). Diarylethene exists as either antiparallel or parallel rotamer. Under light exposure, the antiparallel rotamer undergoes closing of the six-membered ring within its core (Walko and Feringa, 2007). When attached to a polymer chain, cyclization can induce an extension of conjugation structure and rigidification. This leads to a change in the photoelectric properties of the polymer, i.e., oxidation properties of polythiophene, conductivity of polyfluorene, or fluorescence quantum efficiency of a photochromic system (Luo et  al., 2011). In the case of fulgides, UV irradiation results in the closing of the six-­membered ring within its core, which results in the formation of thermally irreversible colored isomers (Yokoyama, 2000). These three examples involve light-­induced intramolecular transitions. Coumarin derivatives undergo reversible intermolecular dimerization to form thermally stable and colorless isomers in response to light (Cardenas-Daw et al., 2012). Dimerization has been applied to adjust the lower critical solution temperature (LCST) of polymers or to stabilize polymersomes by intramolecular or intermolecular cross-linking. Typical examples of irreversible chromophores include photolabile protecting groups (o-nitrobenzyl, coumarin-4-ylmethyl derivatives, and phenacyl esters (Inomata et al., 2000)), pyrenylmethyl, cinnamate derivatives (Ding and Liu, 1998), and 2-naphthoquinone-3-methides (Zhao, 2009; Arumugam and Popik, 2009, 2011a, b; Arumugam et  al., 2012). Photolabile groups are cleaved from the polymer chain upon light exposure. Depending on the position in the chain where the chromophore has been inserted, different light-induced molecular processes can be induced: charge generation in the side groups (Brown et  al., 2009a, b; Cui et  al., 2011), depolymerization and chain shortening (Zhao et al., 2012; Theato, 2011), activation of catalyst and “click” reactant, the formation of active



4.3  Key Types and Properties of Photoresponsive Polymers

91

groups, etc. (Mayer and Heckel, 2006) o-nitrobenzyl ­derivatives undergo light-­induced intramolecular oxidation resulting in the released (uncaged) functionality and a nitrosocarbonyl byproduct, while (­ coumarin-4-yl) methyl leaves a solvent-trapped coumarin byproduct (Goeldner and Givens, 2005). Upon irradiation, phenacyl esters undergo a cleavage by homolytic CO bond scission to give an acryloxy radical and a phenacyl derivative radical. The rapid H atom transfer to the acryloxy radical to yield the carboxylic acid and p-methoxyacetophenone as the photoproduct (Bertrand et al., 2011; Bertrand et al., 2012). 2-naphthoquinone-3-methides generate a highly ­reactive radical that can selectively react with vinyl compounds incorporating an electron-donating group (i.e., oxygen) via very rapid Diels-Alder, adding 2-naphthoquinone-3-methides into the reaction, resulting in the coupling of two species (Arumugam and Popik, 2009, 2011a,b; Arumugam et  al., 2012). Incorporating this group into polymer side chains enables light-induced reactivity, which is useful in photolithography.

4.3  KEY TYPES AND PROPERTIES OF PHOTORESPONSIVE POLYMERS From the viewpoint of chemical structure, several key types of photoresponsive polymers were collected.

4.3.1  Main-Chain Photochromic Conjugated Polymers Photosensitive groups able to switch between a conjugated and a nonconjugated structure (i.e., diarylethenes and spiropyrans) can be introduced into the backbone of a conjugated polymer chain and applied to switch the optoelectronic properties of the material (Luo et al., 2011). The first example of a diarylethene-based backbone photochromic polymer (1, Fig.  4.2) was reported in 1999 (Stellacci et  al., 1999). In the diarylethene open form, it exhibited an absorbance maximum, λmax, at 320 nm. Upon irradiation with a UV light at 313 nm, the open-form polymer was converted to the closed form, which shifted the λmax to >600 nm. The most interesting property of this system was the high quantum yield experimentally obtained for the photoconversion (86%), which was much higher than the 50% value predicted by theory, taking into account the coexistence of the two conformations in equilibrium. The authors attributed the experimental high quantum yield to the stabilization of the active conformation in the polymer structure as a consequence of collective conrotatory motions along the main chain. The switch between closed and open forms also enabled light regulation of the polymer electrochemical response: the closed form of 1 can undergo a reversible redox process whereas the open form

92 4.  Photoresponsive Polymers

FIG. 4.2  Polymers including reversible photosensitive groups in the main chain.



4.3  Key Types and Properties of Photoresponsive Polymers

93

decomposes during the redox process. This allowed a photogated electroswitch. This property has been demonstrated in the d ­ iarylethene-based oligothiophene 2 (Areephong et al., 2008). Terthiophene 2 was polymerized via electrochemical oxidation method. In the presence of light, only the open form was obtained, although both open and closed forms can undergo a reversible redox reaction (Areephong et al., 2008). The possibility of a photoregulated polymerization allowed the authors to deposit a pattern of the conjugated polymer on indium tin oxide (ITO). These results represent an important development for the manufacture of organic electronic devices, although no data on the photoswitching ability of the polymeric film were reported. Following this pioneering work, other two dithienylethene (DE)-based polymers (3, 4) were synthesized via Horner or Wittig reaction (Bertarelli et al., 2004). The inclusion of long alkyl chains in the polymer architecture made these systems soluble in tetrahydrofuran (THF) and facilitated the synthesis of polymers with higher molecular weights (Mn 11600 for 3 and Mn 2702 for 5). In 1999, a different diarylethene-based backbone photochromic polymer (6) was obtained via the Suzuki coupling of dioctylfluorene and diarylethene (Kawai et al., 1999). The resulting polymer had a photocontrollable electrical conductivity: 5.3 × 10−13 S cm−1 in open form and 1.2 × 10−12 S cm−1 in a closed-ring one. The higher conductivity of the closed-ring form was attributed to the extended conjugation pathway throughout the polymer backbone. Substitution of the dioctylfluorene by trimethylsilyl-­substituted phenylene vinylene increased the conductivity of the resulting polymer to 3 × 10−9 (open) and 2.5 × 10−8 (closed) S cm−1 in polymer 7 (Kim and Lee, 2006). Polymers 8 and 9 were also reported to have photoswitchable electrical conductivity (Choi et al., 2005; Kawai et al., 2005). In principle, any photochromic switching unit can be conjugated to a π-electron polymer to allow light-induced changes in the conductivity and optoelectronic properties. Dimethyldihydropyrene, for instance, was introduced into polymer 10 via Suzuki cross-coupling (Marsella et al., 2000). The closed form of polymer 10 allows conjugation through the switching core, whereas the open form has a localized electronic structure. A conductive polymer film was prepared from polymer 10, but solid-state switching could not be observed, presumably due to a slow switching speed. In polymer 11, containing the photochromic switch spirobenzopyran, an absorption band at >500 nm appeared upon irradiation at 365 nm due to the formation of highly conjugated MC (Yang and Ng, 2006). Azobenzenebased main-chain conjugated polymers (12, 13) were accomplished by polycondensation involving Sonogashira-Hagihara cross-coupling and Glaser coupling, respectively. Polymer 12 can undergo photoinduced E→Z isomerization, whereas polymer 13 does not due to the strongly enhanced π, π-stacking interactions of its electron-withdrawing acetylene units (Yu and Hecht, 2015). Polymer 14 could act as “molecular zippers” in the

94

4.  Photoresponsive Polymers

state of thin film in which light-induced E→Z isomerization of 20% azobenzene chromophores triggers a complete disorder of the alkyl chains, which induces the amorphization of the rigid main-chain polymer film (Weber et al., 2015). Main-chain azobenzene conjugated polymers could also show helically folded phenomenon with photoresponsive properties (Sogawa et al., 2013).

4.3.2  Polymers With Photoresponsive Terminal Groups Single photochromic groups attached to the ω-end of a polymer chain have also been used to control the properties of the polymer chains (Roth et al., 2010). Fig. 4.3 presents some examples. The first reported example was polymer 15 containing 2-diazo-1,2-naphthoquinone; a chromophoric unit can undergo Wolff rearrangement to form changed hydrophilic 3-­indenecarboxylates (Goodwin et al., 2005). Amphiphilic 15 can form micelles embedding dye molecules in aqueous solution. Upon irradiation at 350 nm, the change in the hydrophilicity of the terminal group causes dissociation of the aggregates’ release of the encapsulated dye. In a similar approach, polymer 16 was prepared by atom transfer radical polymerization (ATRP) of N-isopropylacrylamide (NIPAM) using an azobenzene derivative substituted with a 2-chloropropionyl group as an initiator (Akiyama and Tamaoki, 2007). Due to the differences in hydrophilicity between the cis (more hydrophilic) and the trans isomers, a cloud-point shift from 32°C to 34°C was induced by switching from trans to the cis isomer upon exposure at 365 nm. A similar effect was observed in poly(oligo[ethylene glycol] methyl ether methacrylate) with a single azobenzene end group (17 in Fig. 4.3) (Roth et al., 2010; Jochum et al., 2009). Similar polymers modified at both ends with azobenzene showed a LCST difference up to 4.3°C between irradiated and nonirradiated solutions (Jochum et al., 2009). It is worth mentioning that this strategy is only effective with low molecular weight polymer chains, and the end-group effects vanish with increasing molecular weight. Recently, several novel examples have been developed. For instance, the tadpole-shaped azobenzene polymer 18 can self-assemble into a large vesicle in aqueous solution, but undergo reversible smooth-curling transformation upon UV irradiation due to the quick trans-cis isomerization of the azobenzene moieties. It can be used for controlled release of drugs (Wang et al., 2015a). Polymers 19 and 20 show interesting photoswitchable fluorescence resonance energy transfer (FRET) properties. 4,4-Difluoro-4bora-3a,4a-diaza-sindacene (Bodipy, donor) and spiropyran (acceptor) are anchored on the terminals of polymers, respectively. The emission of Bodipy is only slightly quenched due to the weak conversion to the open form of spiropyran upon excitation at 360 nm. However, efficient energy transfer from Bodipy to the ring-opened MC form occurs owing to their good spectral overlap of polymers 20 (Kong et al., 2014).

4.3  Key Types and Properties of Photoresponsive Polymers

FIG. 4.3  Polymers containing photosensitive groups on the terminal.

95

96

4.  Photoresponsive Polymers

Helical-shaped bulky alkenes are chiroptical switches and motors that can switch their helical sense in response to light (Feringa et  al., 2000). When attached as end-groups of a helical polymer chain (i.e., poly[nhexyl isocyanate] [PHIC] as represented in 21), they can alternate molecular chirality by light-driven rotation (Pijper et al., 2008). PHIC without a chiral unit adopts an equal ratio of right- and left-handed helical conformation. Addition of chiral end-groups induces the polymer chain to adopt a preferred helical sense (Green et al., 1999). Switching of the helical sense of the terminal group enables reversible control of the preferred helical sense even in the liquid crystalline state. Recently, DE is conjugated to ­oligo(para-phenylene) (polymer 22) for controlling the helicity of its ­assembled state. It was found that the photoisomerization of the chiral DE* terminal moieties prior to assembly leads to a change in the structure helicity (San Jose et al., 2014). In addition to the reversible system, the irreversible nitrobenzyl-based terminal group can also be used to mediate the self-assembly of polymers (de Gracia Lux et al., 2012). Amphiphilic polymers 23 and 24 could self-­ assemble into vesicles to encapsulate the drug doxorubicin (Dox) in aqueous. Upon UV irradiation, the cleavage of the nitrobenzyl group altered the hydrophobicity of the polymers and then triggered a disassembly to release Dox (Cheng et al., 2016; Liu et al., 2017). On the other hand, nitrobenzyl moiety could also be coupled with chain transfer agents (CTA) to prepare end-functionalized homopolymers (Coumes et al., 2016a,b)

4.3.3  Side-Chain Photochromic Polymers Fig.  4.4 presents recent examples of polymers with reversible ­ hotochromic groups incorporated in the side chains. In copolymer 25, p a dimethylaminoethyl methacrylate chain was copolymerized with a ­coumarin-based methacrylic monomer (Zhao et al., 2011a). The LCST of a diluted solution of the copolymer polymer could be modulated between 35°C and 65°C by photocontrolled intramolecular dimerization of the coumarin units upon exposure at 310 nm. The light-triggered formation of chain loops reduced interchain entanglement and caused an increase in the cloud point. Azobenzene- and spiropyran-containing copolymers with light-regulated LCST have also been reported (Zhao et al., 2010). The LCST of spiropyran-containing copolymers increased when the ­chromophore was switched from hydrophobic neutral form to the hydrophilic charged state. The mechanism of azobenzene regulation is less straightforward. In general, bent cis-azobenzene (with a dipole moment of ~4.4 D according to a density functional theory calculation) is more hydrophilic than the trans-azobenzene (0 D) and, therefore, the cloud point of the polymer increases when switching to the cis isomer. However, the contrary effect was observed in a copolymer of N,N´-dimethylacrylamide and

4.3  Key Types and Properties of Photoresponsive Polymers

97

FIG. 4.4  Polymeric systems with reversible photosensitive groups in the side chains.

98

4.  Photoresponsive Polymers

­azobenzene methacrylate. The cis form seems to interact with the neighboring N,N´-dimethylacrylamide unit, and this interaction decreases the LCST. The copolymer mixture 26 exploits the host-guest interaction of azobenzene and cyclodextrins (CD) as responsive engines to induce light-control assembly (Tomatsu et al., 2006). trans-Azobenzene can be selectively encapsulated by CD and, therefore, extend polymer molecular weight via intermolecular cross-linking. Upon UV exposure, the azobenzene unit undergoes trans-cis isomerization and is released from the α-CD site. This leads to an effective decrease in the molecular weight and, consequently, to a decrease of the viscosity of the solution. A similar strategy was used to control sol-gel transition (Tomatsu et al., 2005; Liao et al., 2010). Polymer 27 bears three distinct functional groups including ­azoaromatic, carbazole, and chiral spacer on the side chain, showing high optical activity, additional chiroptical and photoresponsive properties, which gives the possibility of application as a chiroptical switch (Angiolini et al., 2014). Liquid crystal polymers containing azobenzene groups in the side chains have also been reported. Polymers 28-35 (Fig. 4.4) (Fu and Zhao, 2015; Bobrovsky et al., 2014; Bobrovsky et al., 2017; Ryabchun et al., 2017; Kim et  al., 2014a; Cozan et  al., 2016) are interesting examples in which light-triggered conformational changes of the azobenzene units led to nematic/smectic-isotropic phase transitions (Yu and Ikeda, 2011; Li et al., 2003). These systems will be discussed in detail in Section 4.3. Isomerisation of spiropyran moieties (36 in Fig. 4.4) introduced in the side chain have also been used to modulate polymer solubility in an aqueous environment (Byrne et al., 2011). To avoid the incompatibility of the spiropyran derivative’s feature and ATRP technique, copper-catalyzed cycloaddition (click chemistry) was employed to obtain well-defined spiropyran functionalized polymer 37, which showed similar behavior with comparable ring-closure kinetics and photostability comparing with the star-like spiropyran polymers (Ventura et  al., 2014). Comb-shaped graft copolymer 38 at two side-chain lengths featuring polyacrylonitrile backbones and photoreactive side chains could be coated on the porous film to achieve self-cleaning properties. Before any photo treatment, the as-coated membrane surface comprises mostly hydrophobic spiropyran groups that allow the adsorption of organic solutes such as proteins on the membrane surface. Upon UV irradiation, the spiropyran groups are converted into hydrophilic MC groups, which leads to the release of adsorbed molecules and the full recovery of the initial water flux (Kaner et al., 2017). In addition to hydrophobicity, the sypropyran-based polymers could also be exploited as the photochromic acceptor of fluorescent moieties (Keyvan Rad et al., 2016), fluorescence photomodulation, or cell imaging (Chamberlayne et al., 2014; Lee et al., 2014). Recently, ­sypropyran-based copolymers were



4.3  Key Types and Properties of Photoresponsive Polymers

99

modified on upconversion nanoparticles via self-assembly and used for photocontrolled release by near-infrared (NIR) light (Xing et al., 2015). Photoresponsive polymers have great potential in photoswitchable molecular devices and optical memory storage systems. Polymer 39 contains 2, 5-demethyl (thienyl) ring with a photochromic ligand on the side chain (Price and Ragogna, 2013). It shows a very high recyclability with almost no decomposition after five cycles. In addition to this example, a series of photoresponsive and full-colored fluorescent-conjugated copolymers have been prepared by combining phenylene- and thienylene-based main chains with photochromic DE side chains. They show photoswitchable fluorescence in both solution and film states through light-controlled photoisomerization (Watanabe et  al., 2015). Copolymer 40 has both DE and fluorene moieties on side chains. Its fluorescence intensity significantly decreased with increasing photocyclizaiton conversion of the DE due to the quenched fluorescence of many fluorescence moieties by one closedring DE moiety (Nakahama et al., 2017). Fig.  4.5 shows some examples of reported polymer systems carrying irreversible photochromic groups in side chains. Polymer 41 bears a dimethylphenylsulfonium triflatecan unit as a photoacid generator, which undergoes homolytic cleavage followed by hydrogen abstraction and rearrangement to generate triflic acid after expose with UV light of 254 nm (Brown et  al., 2009b). The resulting strong acid catalyzes the hydrolysis of neighboring t-butyl esters and leads to the formation of poly(methacrylic acid) (PMAA). Thin films of this polymer show a light-induced wettability change. Simaliar photoacid generator can be incorporated into a polymer (42) for killing cell (Sumaru et  al., 2013) and photolithography (Liu et al., 2014a). In a different approach, photocleavable units attached to ionizable carboxylic or amine groups were exploited to change the solubility and wettability of polymer brushes (43, 44, 45) (Cui et  al., 2011, 2012a; Dinu et  al., 2016). Polymer 43 presents one example where the 4,5-­ dimethoxy-2-nitrobenzyl (NVOC) photocleavable protecting group is attached to the side chain carboxylic groups of a PMAA chain (Brown et al., 2009a). Light irradiation removed the NVOC group and released free carboxylic groups. When a surface covered with these polymer brushes was irradiated through a mask, a surface pattern with zones with different wettabilities was generated. Regulation of the exposure dose allowed the development of different wetting states as a consequence of different photoconversion degrees (Cui et al., 2012a). Based on this idea, a series of photolabile polymers with pendent nitrobenzyl groups (polymer 46, 47) have been developed and applied in micelles/nanoparticles preparation and drug/dye encapsulation (Olejniczak et  al., 2015; Zhao et  al., 2016; Shen et al., 2017). Polymer 45 also bears the NVOC-protected cationic moiety, but it was used to generate an intramolecular ionic pair for phototriggered cell uptake and slow releasing (Dinu et al., 2016). Both polymers 44 and 48

100 4.  Photoresponsive Polymers

FIG. 4.5  Polymers with photolabile groups on the side chains.



4.3  Key Types and Properties of Photoresponsive Polymers

101

have protected amine group. Polymer 44 was applied to light-control the selectively of ionic permeation when incorporated into the pores of membranes (Cui et al., 2011; Brunsen et al., 2012) whereas polymer 48 was used to make stable patterns by light-activating amine moieties to react with featuring pentafuorophenyl ester (Zhao and Theato, 2013). Polymer 49 represents a strategy to photoregulate the formation of supramolecular polymers by attaching a caged 2-ureido-4-pyrimidone (UPy) in the side chain (Foster et  al., 2009). UPy can dimerize via self-­ complementary quadruple H-bonding with high affinity (De Greef et al., 2009; Sijbesma et al., 1997). In polymer 49, UPy units were modified by reaction with the photolabile o-nitrobenzenyl. This modification ­inactivated the H-bonding acceptor. Irradiation of a diluted solution of polymer 49 removed the cage and induced intramolecular cross-linking by H-bonding, resulting in the formation of single molecular nanoparticles. In contrast, the o-nitrobenzyl chromophore in polymer 50 was designed as photodegradable cross-linker, which allowed UV-mediated depolymerization. This system underwent softening by 20%-30% upon irradiation at a dose tolerated by living cells (Frey and Wang, 2009). Similar photodegradable nanoparticle 51 was designed for controlling protein delivery. It was prepared by an emulsion copolymerization of 2-(dimethylamino) ethyl methacrylate and a photoliable o-nitrobenzyl diacrylate cross-linker. In a demonstration with bovine serum albumin and green fluorescent protein as model proteins, the nanoparticles show a photo-triggered release in presence of cells (Jiang et al., 2015).

4.3.4  Side-Chain Photochromic Block Copolymers Photosensitive block copolymers have been intensively studied due to their self-assembling properties and drug delivery applications. Several recent reviews (Zhao, 2009; Schumers et al., 2010a; Zhao et al., 2012) have been recently published and, therefore, this section only reviews recent developments. Fig. 4.6 presents diblock copolymers with reversible photoresponse. Most of these systems have been developed to photocontrol micelle formation by changing the hydrophilic-hydrophobic balance in the chain. Pioneering work was carried out in Zhao’s research group. They used ATRP to synthesize polymer 52, a block copolymer containing one random poly(t-butyl acrylate-co-acrylic acid) sequence and a poly-(methacrylate) block with azobenzene chromophores in the side chains (Tong et al., 2005). The polymer self-assembled into core-shell micelles or vesicles when the azobenzene adopted the trans form, an almost symmetrical structure with a near-zero dipole moment (no charge separation). Illumination of the micellar solution with UV light (360 nm) switched the trans azobenzene to its cis-isomer with a dipole moment of ~4.4 D, which resulted in a large increase in the polarity of the hydrophobic block. As a

102 4.  Photoresponsive Polymers

FIG. 4.6  Block copolymers with reversible photosensitive groups on the side chains.



4.3  Key Types and Properties of Photoresponsive Polymers

103

result, the azobenzene modified block was no longer hydrophobic enough to preserve the micellar association, and micelles dissociation occurred. By exposing the system to visible light (440 nm), the trans isomer was favored and micelles reformed. The transition can also be used to trigger a reversible sol-gel transition in triblock polymers system like 53 (Ueki et  al., 2015). This approach can be applied to any photochromic molecule with isomers with different polarities, like in the case of polymer 54 consisting of hydrophilic PEO block and methacrylate ester block with photosensitive spiropyrane on side chain (Lee et al., 2007). With its amphiphilic structure, 54, 55 (Wang et al., 2015b) self-assembled into micelles. Under UV irradiation, conversion of the hydrophobic spiropyran moieties into their hydrophilic zwitterionic MC counterparts occurred and, consequently, the micelles disassembled. Recovery of the micells was triggered by exposure to visible light (620 nm). Polymer 56 could act as the soft interface to control wettability and cell adhesion by alternating irradiation using UV and visible light (He et al., 2017). Polymer 57 formed micelles with reversible cross-links (Jiang et al., 2007; Babin et al., 2008), and polymer 58 formed liquid crystalline phases with photoswitchable orientation in the solid state (Yu et al., 2006). Fig. 4.7 exhibits the examples of diblock copolymers with irreversible photoresponse. Polymer 59 contains photolabile-protecting groups attached to carboxylic groups in the side chains (Jiang et  al., 2005). Upon UV irradiation, photosolvolysis of the pyrenylmethyl ester occurs, 1-­ pyrenemethanol is cleaved from the polymer chain, and carboxylic acid groups are released. As a consequence, the hydrophobic block turns into a hydrophilic PMAA block. Core-shell micelles formed by 59 disappeared after irradiation with UV light of 365 nm. This design was further validated with other chromophores (polymers 60-63) (Babin et al., 2009; Jiang et  al., 2006; Schumers et  al., 2012). Polymer 64 was prepared by a ring-opening polymerization of 3-methyl-3-nitrobenzyl-trimethylene carbonate bearing numerous nitrobenzene photolabile groups. It can self-­ assemble into spherical micelles upon heating in an aqueous solution. Upon light irradiation, a burst occurs on the particles, which could be used to release the encapsulated drug (Fang et al., 2015). Polymer 65 has a side chain containing disulfide spacer and pendant o-nitrobenzyl t­hioether group. This side chain can respond to both Glutathione and light, which allows for synergistic control release (Sun et al., 2015). Both polymer 66 and 67 also show light-activated release. In polymer 66, a photocleavable linkage is used to connect a cationic group that could effectively complex pDNA into salt-stable polyplexes with appropriate sizes (Green et  al., 2014). Irradiation-induced cleavage in 66 leads to facilitative pDNA release and efficient nucleic acid delivery. Compared to the cationic group, the functional retinoic acid group could be covalently attached to polymer 67 through a photosensitive nitrobenzene linker (Gupta et al., 2017). As a

104 4.  Photoresponsive Polymers

FIG. 4.7  Block copolymers with photolabile groups on the side chains.



4.3  Key Types and Properties of Photoresponsive Polymers

105

result of the amphiphilic structure, it could self-assemble into micelles. When exposed to a light of 365 nm, the nitrobenzene linker was efficiently cleaved and consequently, the retinoic acid released. In addition, nitrobenzyl group was recently integrated into polypeptide block polymers for the fabrication of photoresponsive nanomedicine (Liu and Dong, 2012), preparing thin films with nanostructures (Schumers et al., 2012), and prodrug releasing system (Hu et al., 2013). In polymer 68, the polystyrene (PS) and poly(ethylene oxide) blocks were connected by a photolabile o-nitrobenzyl linker (Theato, 2011). Light exposure cleaved the polymer backbone and separated the hydrophobic and hydrophilic blocks. This process was successfully carried out in both liquid and solid states. Thin films of polymer 68 can be annealed to generate a vertically aligned cylindrical morphology. After UV irradiation followed by methanol/water washing, the film leads to a nanoporous PS structure (Kang and Moon, 2009). This kind of diblock copolymer can be obtained either by ATRP polymerization or by copper(I)-catalyzed azidealkyne cycloaddition of the two presynthesized blocks. The latter method is preferred if the composition of the copolymer needs to be tailored (Han et al., 2012; Schumers et al., 2010b).

4.3.5  Photosensitive Dendritic Polymers Aida et al. reported the first azobenzene-core dendrimer in 1997 (Jiang and Aida, 1997). The phenyl ring on the periphery of the dendrimer can harvest the light at long wavelengths and transfer the energy into the core via a multiphoton absorption process, leading to the trans-cis transition of azobenzene. Reported work on light-harvesting dendrimers has been recently reviewed (Bradshaw and Andrews, 2011) and, therefore, we do not include them in this chapter. Azobenzene-based dendrimers or dendrons constitute a big family (Chen et al., 2012a; Deloncle and Caminade, 2010). The chromophore can be integrated into the molecular periphery, at internal positions, or in the core depending on the expected properties (Zhang et al., 2011b). Reported examples have been reviewed in 2010 (Deloncle and Caminade, 2010), and we collect here only relevant systems reported since then (Fig. 4.8). Dendrimer 69 contains azobenzenyl-linked polyphenylenes (Nguyen et al., 2011). Upon irradiation at 365 nm, the extended six dendrons curled toward the core as a consequence of the trans-cis transition of azobenzenyl spacers, resulting in high density closed structure. Because of its rigidity, the dendrimer can retain guest molecules in the closed form and release them in response to light. Such photoinduced size change is also found in carbosilane dendrimers bearing 4-phenylazobenzonitrile units (Koyama et al., 2009). Dendrimer 70 is the first water-soluble dendrimer that responds to light stimulus (Hayakawa et al., 2003). UV irradiation in

106 4.  Photoresponsive Polymers

FIG. 4.8  Photosensitive dendrimer and dendritic polymers.



4.3  Key Types and Properties of Photoresponsive Polymers

107

aqueous solution induces unusual irreversible trans-cis isomerization and leads to 100% of cis isomer. Interestingly, the fluorescence maximum of the stilbene core in this dendrimer shifted from 424 nm to 411 nm and then to 389 nm by increasing generation from G1 to G3. This shift was attributed to the isolation of the stilbene core by the hydrophobic dendron, which decreases the interaction between the stilbene and water and consequently, reduces the stabilization of the excited state of the stilbene core by water. Stilbene was also integrated into the branch, but the p ­ hoto-responsive behavior was not demonstrated (Cano-Marín et al., 2005). Hyperbranched polymer 71 obtained by modification of hyperbranched poly(ether amine) with 4-phenylazophenyl glycidyl ether self-assembled in aqueous solution at 80°C into nanoparticles with 10-18 nm diameter (Yu et al., 2010). Light exposure switched trans-azobenzene to the cis form, which caused an increase in the LCST of the system of 5.3°C. Linear-dendritic polymer 72 has an amphiphilic structure with azobenzene in the periphery and thus could encapsulate of both hydrophobic and hydrophilic molecules through forming polymeric vesicles in water. Light-induced trans-to-cis isomerization of the azobenzene moieties with a low dose can lead to the release of the loaded molecules (Blasco et  al., 2013a). In the case of amphiphilic linear-dendritic polymers 73 in which the azobenzenene moieties were diluted by alkyl chains, the trans-to-cis photoisomerization rate is faster (Blasco et al., 2013b). Polymer 74 has a hydrophilic core and an ­azobenzene-containing hydrophobic shell. With light stimulus, the ­polymer is able to encapsulate anionic guests from an aqueous solution to an organic layer (Cao et al., 2015). These polymers may be good candidates to develop drug delivery systems because they can be easily synthesized and possess intrinsic “core-shell” structures. In addition to delivery systems, photochromic azobenzene moiety could also be applied to prepare liquid crystalline and nonlinear optical materials (Kim et al., 2014b; Yang et al., 2016). Spiropyran is the other chromophore frequently incorporated into hyperbranched polymers. Polymer 75 is one of the recent examples (Chen et al., 2012b). It could self-assemble to biocompatible micelles with an average diameter of 186.3 nm. After 5 min of UV irradiation, the diameter of the micelles decreased gradually to about 100 nm, which is ascribed to the transformation of hydrophobic spiropyran to hydrophilic MC. This study provides a convenient way to construct smart nanocarriers for controlled release and re-encapsulation of hydrophobic drugs. Irreversible photoresponsive dendritic polymers have mainly been developed for photodegradation properties (Pasparakis et  al., 2012; Kevwitch and McGrath, 2007; Kevwitch and McGrath, 2001; Kevwitch and McGrath, 2002). Fig. 4.9 displays three examples containing photolabile groups at the core, the branch, or the periphery of the dendritic structures. Compound 76 is the first reported caged dendritic structure with a LeuLeuOMe unit on the periphery connected by a photolabile spacer to

108 4.  Photoresponsive Polymers

FIG. 4.9  Dendritic polymers with irreversible photosensitive groups.



4.3  Key Types and Properties of Photoresponsive Polymers

109

the core (Watanabe et al., 2000). Upon irradiation, about 50% LeuLeuOMe was released. Polymer 77 contains a photolabile core that converts the original polymer into smaller dendrons by photodegradation under UV exposure (Smet et al., 2000). Dendrimer 78 contains photolabile o-nitrobenzyl groups at the periphery, which formed a hydrophobic rigid shell that can prevent the diffusion of encapsulated salicylic acid (Li et al., 2010). UV exposure cleaved the chromophore and left an amine-terminated dendrimer, which breaks the shell. As a consequence, the release of encapsulated molecules in the dendrimer could be significantly improved. Polymer 79 has a linear-dendritic structure with a photolabile spacer. It was designed for light-triggered synergistic effect for releasing dyes (Kalva et  al., 2015). Diazonaphthoquinone was modified on linear-dendritic amphiphiles 80, the structure of which could undergo Wolff rearrangement and change from hydrophobic to hydrophilic triggered by both UV and NIR light. This transformation could increase the release rate of drug molecules stored in the self-assembled micelles of 80 and then kill the cells in an NIR-triggered manner (Sun et al., 2014).

4.3.6  Photosensitive Supramolecular Polymers In 1998, supramolecular polymers with photocontrol molecular weights were reported for the first time (Fig.  4.10) (Folmer et  al., 1998). These are formed by the self-assembly of telechelic polymers terminated into 2-­ureido-4-pyrimidone (UPy) units. UPy can dimerize by quadruple H-bonding with high affinity (Km of 2.2 × 106 M−1 in chloroform), building up long chains (Sijbesma et  al., 1997). A chloroform solution of this polymer (81) behaves like a solution of a conventional covalently linked polymer. The degree of polymerization and, therefore, the viscosity, shearing effects, and viscoelastic properties are controlled by the ratio of monofunctional UPy added to the solution, which end-caps the growing chains (Sijbesma et  al., 1997). By protecting the UPy end-groups or the monofunctional UPy with the photolabile group o-nitrobenzyl, the ability of UPy to dimerize is prevented. As a consequence, the polymerization and depolymerization processses can be tuned by irradiation with UV light. This phototriggering H-bonding strategy was applied to synthesize single molecule nanoparticles (polymer 49) and prepare light-responsive hydrogels with self-healing ability (Berda et al., 2010). Recently, we synthesized a novel silanizing agent containing o-nitrobenzyl protected UPy (82) for studying UPy dimerization underwater (Cui and del Campo, 2012). H-bonding cross-linking interaction is widely used to build higher structures in biological systems such as protein and nucleic acid but was rare in the synthetic systems underwater because of the disturbance of water molecules. To fully study the flexibility of the dimerization of UPy underwater, 82 was modified on a substrate to generate a photo-activated surface. In an in-situ comparison experiment, it was found that UPy-based

110

4.  Photoresponsive Polymers C13H27 N

O

N

C13H27

H

H

O

O

N

N

H

H

C6H12

N

N

H

H

(A)

N

O

n

NO2

N O

N

O N H

C13H27

C13H27 NO2

N

N

CH2O

N

N

O N

N

H

H

C4H9

O

N

H

O

N

N

H

H

C4H9

81

O N H

Si

O

O

82 NH O

Silicon

(B)

caged UPy-modified

UV

Silicon

O

N

N H

N H

UPy-modified

FIG. 4.10  (A) Supramolecular polymer consisting of two UPy units and monofunctional UPy unit with and without a photolabile protected group. (B) Silanizing agent containing o-nitrobenzyl protected UPy compound and its dimerization underwater. Adapted with permission from Folmer, B.J.B., Cavini, E., Sijbesma, R.P., Meijer, E.W., 1998. Chem. Commun. 1847–1848; Cui, J., del Campo, A., 2012. Chem. Commun. 48, 9302–9304.

copolymer can stably bond to the activated substrate even underwater. Based on this observation, UPy-based self-healing hydrogel was prepared (Cui and del Campo, 2012; Jeon et al., 2016). The sample idea was further expended to photoresponsive hydrogel thin film (Cui et al., 2013b). Another strategy to design photoresponsive supramolecular polymers is based on the host-guest interactions. Polymer 83 shows a typical ­system with stiff-stilbene as guest and bispillar[5]arene as host. Stiff-stilbene can transfer between Z and E configuration under 360 and 387 nm irradiation, respectively. The Z-configuration is favorable for forming a self-­ complexing structure with the bispillar[5]arene, whereas E-one prefers a linear supramolecular structure (Wang et al., 2014a). The formation and disassembly of the supramolecular polymer were reversible by alternating irradiation between 387 nm and 360 nm light (Fig. 4.11) (Wang et al., 2017). The same idea was also demonstrated by the system of stiff-stilbene and pillar[6]arene/pillar[7]arene (Xia et al., 2016; Chi et al., 2015) or used to prepare a multiresponsive gel (Xu et al., 2013). Fig.  4.12 presents a different example of photosensitive supramolecular polymers based on the host-guest interaction between CD- and adamantine-terminated dimers (Polymer 84) (Kuad et  al., 2007). The ­ ­stilbene spacer undergoes a light-induced reversible trans-cis photoisomerization and changes the orientation and the distance between CD units. In trans conformation, the monomers self-assemble into dimers or short ­oligomers. Upon irradiation with UV light of 350 nm, stilbene is converted into its cis conformation and allows the formation of supramolecular linear polymers with high molecular weights. A similar host-guest



4.3  Key Types and Properties of Photoresponsive Polymers

111

FIG. 4.11  (A) Chemical structures of light-responsive stiff-stilbene-bridged symmetrical guests (Z-G/E-G). (B) Photocontrolled assembly and disassembly of an AA/BB supramolecular polymer. Adapted with permission from Wang, Y., Sun, C.-L., Niu, L.-Y., Wu, L.-Z., Tung, C.-H., Chen, Y.-Z., Yang, Q.-Z., 2017. Polym. Chem. 8, 3596–3602.

interaction was exploited to prepare photoresponsive supramolecular hyperbranched polymer 85 using an azobenzene dimer and a β-CD trimer (Fig. 4.12B) (Dong et al., 2011). Light-induced trans-to-cis transition of the azobenzene results in a bended conformation of the guest and disfavored its host-guest interaction with β-CD. As a consequence, depolymerization occurs. Recent studies on azobenzene-based polymers with different terminal groups such as chiral binaphthyl (Sun et  al., 2013), adamantine (Liu et  al., 2014b; Nachtigall et  al., 2014), and other functional moieties (Xia et  al., 2014; Mazzier et  al., 2014; Endo et  al., 2016; Concellón et  al., 2016; Wang et  al., 2016a) have led to various novel multifunctional

112

4.  Photoresponsive Polymers

FIG.  4.12  (A) Structures of stilbene bis(β-CD) dimer and C3 guest dimer and the photoswitched transition between dimer and polymer. (B) Schematic representation of the photocontrolled polymerization and depolymerization of a β-CD3/Diazo supramolecular hyperbranched polymer based on host-guest interactions. (C) Chemical structures of the azobenzene-bridged pillar[5]arene dimer (H4) and switching between the assembly and disassembly of a supramolecular polymer by alternating between UV and visible light irradiation. A: Adapted with permission from Kuad, P., Miyawaki, A., Takashima, Y., Yamaguchi, H., Harada, A., 2007. J. Am. Chem. Soc. 129, 12630–12631; B: Adapted with permission from Dong, R., Liu, Y., Zhou, Y., Yan, D., Zhu, X., 2011. Polym. Chem. 2, 2771–2774; C: Adapted with permission from Ogoshi, T., Yoshikoshi, K., Aoki, T., Yamagishi, T.A., 2013. Chem. Commun. 49, 8785–8787.

­ hotoresponsive supramolecular polymers. One interesting example is p to trigger the reversible folding of the linear supramolecular polymer 86 (Fig.4.12C) (Adhikari et al., 2017). It was constructed via step-growth polymerization of azobenzene-bridged pillar[5]arene dimer (host molecule) and ­di-­pyridinium (guest molecule) (Ogoshi et al., 2013; Yang et al., 2014a), and its degree of polymerization increases exponentially with the concentrations of the building blocks. By using UV and visible light, the assembly and disassembly of the supramolecular 86 can be controlled. Tweezer/guest complexation is a novel method to build supramolecular polymer systems. Fig. 4.13 shows an example with ­bis[alkynylplatinum(II)]

4.3  Key Types and Properties of Photoresponsive Polymers

113

FIG. 4.13  Schematic representation for the formation of supramolecular polymer networks via hydrogen bond-assisted molecular tweezer/ guest complexation.Adapted with permission from Gao, Z., Han, Y., Chen, S., Li, Z., Tong, H., Wang, F., 2017. ACS Macro Lett. 6, 541–545.

114

4.  Photoresponsive Polymers

terpyridine as molecular tweezer and trans-azobenzene as recognition motif. In the assembled state 87, intermolecular O−H···N hydrogen bond forms, which enhance the cross-linking strength. Taking advantage of the trans-cis transition of the azobenzene moieties, the supramolecular network is photoresponsive (Gao et al., 2017).

4.4 APPLICATIONS Light-induced changes at polymer level can be reflected on the macroscopic level of material properties, which has been used in various applications.

4.4.1  Controlled Drug Delivery Polymer micelles or vesicles formed through self-assembly of photoresponse block copolymers can be applied as carriers for controlled drug delivery. Fig.  4.14 schematically illustrates the general mechanism for photocontrolled polymer micelles; light exposure induces solubility changes in the block modified with the photochromic group and, as a consequence, the micelles disassemble (Zhao, 2009; Zhao, 2012). Fig. 4.15 present a typical example of reversible photoregulated micelles (Lee et al., 2007). The diblock copolymer 54 self-assembles into micelles with the hydrophobic spiropyran-based block in the core. Light exposure switches hydrophobic spiropyran to its charged MC form, which enhances the solubility of polymer chains and, consequently, the micelles disassemble. When the micelle was loaded with a hydrophobic dye, UV exposure allowed the release of the dye, which could be re-entrapped by irradiation with visible light. Moreover, other functional groups can be incorporated into the s­ piropyran-related polymers to design multiresponsive micelles (Lee et al., 2014; Son et al., 2014; Shen et al., 2015). Fig. 4.16 shows a specific example of using photoresponsive polymers to control drug delivery (Shao et  al., 2014; Ji et  al., 2013; Barman et  al., 2015). In this system, coumarin was incorporated into linear dendritic copolymers to form photocross-linkable nanocarriers to load drug molecules. Obtained nanocarriers can be triggered to degrade to release the drug molecules in vivo by taking advantage of the photoinduced reversible dimerization of coumarin. In addition, azobenzene was another good host-guest inclusion candidate for self-assembling stable vesicles and rapid releasing of anticancer drugs (Xia et al., 2014; Li et al., 2014a; Zhang et al., 2015a; Xiao et al., 2015). Light excitation in the NIR is more convenient for drug delivery because it is able to penetrate the tissue and it does not cause cell damage. Although two-photon excitation can, in principle, extend the activation



4.4 Applications

115

FIG. 4.14  Schematic illustration of various types of light-responsive block copolymer micelles. Adapted with permission from Zhao, Y., 2012. Macromolecules 45, 3647–3657; Gohy, J.F., Zhao, Y., 2013. Chem. Soc. Rev. 42, 7117–7129; Yan, Q., Han, D., Zhao, Y., 2013. Polym. Chem. 4, 5026–5037.

wavelength of a chromophore to the NIR region, most chromophores have low two-photon-absorption cross-sections, and the photoreaction occurs with low efficiency and requires high-power femtosecond pulse lasers. To overcome this limitation, Lanthanide-doped upconverting nanoparticles (UCNPs) have been proposed for building NIR light-response micelles (Fig. 4.17) (Yan et al., 2011). UCNPs can absorb NIR light and then convert it into higher-energy photons in the UV and visible regions, which is absorbed by the photochromic moieties of polymer 59 in the core-forming block. As a consequence of the photocleavage reaction, the core becomes hydrophilic, and the micelles dissociate and release their payload. This strategy can be applied to different photoliable groups and material systems (Song et al., 2013; Yu et al., 2015).

116

4.  Photoresponsive Polymers

FIG. 4.15  AFM images of (A) a micellar solution spin-coated on mica (a1 and a2 are height and volume distributions of micellar aggregates, respectively), (B) dissociated micelles after 30 min UV exposure (365 nm). (C) Reformed micelles after subsequent visible light (620 nm) exposure for 30 min, and (D) for 120 min (d1 and d2 are height and volume distributions of reformed micellar aggregates, respectively). Adapted with permission from Lee, H.-i., Wu, W., Oh, J.K., Mueller, L., Sherwood, G., Peteanu, L., Kowalewski, T., Matyjaszewski, K., 2007. Angew. Chem. Int. Ed., 46, 2453–2457.

4.4.2  Functional Micropatterns Site-selective exposure of thin films of photosensitive polymers using masks or scanning lasers can be applied to make functional patterns onto substrates, like the examples presented in Fig.  4.18 (Anastasiadis et  al., 2008; Barille et  al., 2011; Matsumoto et  al., 2008; Kim et  al., 2012). Thin



4.4 Applications

117

FIG.  4.16  Illustration of the cross-linking and decross-linking process of the c­ oumarin-containing photosensitive phase-segregated micelle nanocarriers. Adapted with permission from Shao, Y., Shi, C., Xu, G., Guo, D., Luo, J., 2014. ACS Appl. Mater. Interfaces 6, 10381–10392.

films of the main-chain conjugated polymer 7 spin-coated on an electrode generate a color conductive pattern when irradiated through a mask with parallel micrometric stripes (Fig.  4.18A). The dark areas represent the masked, highly conductive region. The bright areas correspond to the light-exposed resistive region where diarylethene adopt open forms with lower conductivity (Kim and Lee, 2006). Azobenzene-based polymers have been repeatedly used for creating surface microreliefs or graftings when an interfering laser is used for illumination (Gong et al., 2011; Lomadze et al., 2011; Wang et al., 2011). Fig. 4.18B presents atomic force microscopy (AFM) images of the surface relief grating formed on films of epoxy-based polymers containing azobenzene groups at side chains after irradiation at 488 nm. Light exposure with high-energy interfering laser induces mass transport in the polymeric film, which involves the scission of covalent bonds and mass transition. The mechanism of the laser-induced periodic surface structure is still unclear, but the investigation of these surface relief graftings still attract great attention (Rocha et al., 2014; Koskela et al., 2014; Rianna et al., 2015; Landry et al., 2017; Zong et al., 2016). One of the interesting examples published recently is to create hierarchical surface patterns on azo-containing multilayer films. Large-area surface wrinkling was induced in the multilayer-based film/substrate system through external stimuli of heating/cooling processing. When the formed wrinkle morphologies were selectively exposed to visible light through copper grids, the wrinkle wavelength reduced gradually with the light irradiation and finally reached a saturated value. In contrast, the unexposed region evolved into highly ordered wrinkles, leading to microstructure patterns (Zong et al., 2016). Compared to the reversible chromophore system, thin films made from the irreversible photoresponsive system of polymer 43 can ­generate

118

4.  Photoresponsive Polymers

FIG.  4.17  (A) Photosensitive micelles that encapsulate upconversion nanoparticles (UCNPs) and allow excitation in the NIR; (B) Photo-triggered reaction occurred in the polymers of the micelles. Adapted with permission from Yan, B., Boyer, J.-C., Branda, N.R., Zhao, Y., 2011. J. Am. Chem. Soc. 133, 19714–19717.

FIG.  4.18  Patterns of (A) polymer 7, (B) azobenzenyl polymer, and (C) polymer 22. Adapted with permission from Kim, E., Lee, H.W., 2006. J. Mater. Chem. 16, 1384–1389; Gong, Y.-H., Li, C., Yang, J., Wang, H.-Y., Zhuo, R.-X., Zhang, X.-Z., 2011. Macromolecules 44, 7499–7502; Lomadze, N., Kopyshev, A., Rühe, J.r., Santer, S., 2011. Macromolecules 44, 7372– 7377; Wang, X., Yin, J., Wang, X., 2011. Macromolecules 44, 6856–6867; Cui, J., Huong, N.T., Ceolín, M., Berger, R.d., Azzaroni, O., del Campo, A., 2012. Macromolecules 45, 3213–3220.

a chemical pattern with regions of different wettability upon light exposure (Fig.  4.18C). The light-induced release of the o-nitrobenzyl photolabile protecting group from the polymer structure generates a polyelectrolyte and, consequently, makes exposed regions hydrophilic and pH-sensitive (Cui et al., 2012a). In contact with water or in a humid



4.4 Applications

119

a­ tmosphere, the exposed regions can uptake water and swell, generating a surface relief with a pH-tunable height difference between irradiated and nonirradiated regions. o-Nitrobenzyl-based photoresponsive polymers are also good candidates for making hydrogel and cell patterns (Radl et  al., 2017; Li et  al., 2014b; Tsang et al., 2015; Ding et al., 2017). Fig. 4.19 shows a typical example that combines both photoinduced formation of thiol-ene network and photoinduced degradation of nitrobenzyl spacers. The hydrogel was prepared by photoinduced “click” reactions with visible light (λ > 400 nm), which does not induce any photocleavage of the nitrobenzyl links. Upon irradiation of UV light (λ < 400 nm), the cleavage of covalent links occurs. It can be used to create the second pattern (Fig. 4.19A and B) (Radl et al., 2017). Furthermore, the functional o-nitrobenzyl-ester-based polymres could be modified on the surface of the material and used for cell patterns. A novel strategy for constructing cell patterns on titanium substrates have been developed by combining o-nitrobenzyl with UCNP (Fig. 4.19C and D). The o-nitrobenzyl was used as a photolabile spacer to link the bioadhesive ligand arginine-glycine-aspartic acid (RGD) to UCNP. Upon an irradiation of the NIR light (980 nm) through a photomask, the UCNP could transfer NIR light into UV light in situ, which results in the photocleavage and detachment of the unsheltered cells for the formation of cell pattern (Ding et al., 2017). Photoreactive chromophores incorporated into polymer films can be used for inducing site-specific surface reactions and generation of chemical patterns. The chromophore 2-napthoquinone-3-methide generates highly reactive radicals upon exposure, which can selectively react with vinyl groups with electron-donating substituents, or turn back to their ground state and regenerate the photochemical precursor (Fig.  4.20) (Arumugam and Popik, 2009, 2011a,b; Arumugam et  al., 2012). This chromophore has been incorporated into the side chain of a poly(N-hydroxysuccinimidyl 4-vinylbenzoate) backbone and used for the surface immobilization of different species. An azide and an alkyne-terminated vinyl ether were photopatterned onto the polymer surface and then reacted with an alkyne or azide-terminated fluorophore using the azide-alkyne click-reaction (Arumugam et  al., 2012). A fluorescent pattern was obtained. This simple method can be extended to attach any molecule or biomolecule to the surface with a high yield and a good selectivity. In addition, copolymers containing 2-­ methacryloyloxyethyl phosphorylcholine (MPC) and N-methacryloyl-(L)-tyrosinemethylester (MAT) groups were also used for region-specific immobilization of proteins and cells (Tanaka et al., 2017). When the copolymer-P(MPC/MAT) was modified on the silicon or gold surfaces, photoinduced oxidation of the MAT units generates catechol groups that could react with the amine or the thiol groups

120 4.  Photoresponsive Polymers

FIG. 4.19  Photoinduced formation and light-triggered cleavage of thiol-ene networks for the design of switchable hydrogel patterns in (A) and (B). The process of the construction of cell patterned surface in (C) and (D). Adapted with permission from Radl, S.V., Schipfer, C., Kaiser, S., Moser, A., Kaynak, B., Kern, W., Schlögl, S., 2017. Polym. Chem. 8, 1562–1572; Ding, T., Yang, W., Luo, Z., Liu, J., Zhang, J., Cai, K., 2017. Mater. Lett. 209, 392–395.



4.4 Applications

121

FIG.  4.20  Generation of chemical patterns with photoreactive polymers: PhotoDiels−Alder surface anchoring followed by Azide-Alkyne Click-Reaction to immobilize Fluorescent Dyes. Adapted with permission from Arumugam, S., Orski, S.V., Locklin, J., Popik, V.V., 2012. J. Am. Chem. Soc. 134, 179–182.

of proteins. Compared to the high adhesion of the irradiated region, the nonUV-­ irradiated P(MPC/MAT) surface is protein-­ repellent. Therefore, cell/protein patterns can be made.

4.4.3  Responsive Hydrogels Stimulus-responsive polymeric hydrogels are useful materials with applications in drug/gene delivery, photography, paints/coatings, scaffolds for tissue-engineered prostheses, biosensors, or actuators (Chaterji et  al., 2007; Tokarev and Minko, 2010; Lyon et  al., 2009). In most cases, the stimulus causes a molecular change (ionization, cross-linking) that affects the swelling degree of the hydrogel. In 1967, Lovrien et  al. suggested a strategy to prepare photoresponse hydrogels with photochromic dyes (Lovrien, 1967), and this was first experimentally realized by Van der Veen and Prins in 1971. A poly(2-hydroxyethyl methacrylate) hydrogel was mixed with sulfonated bisazostilbene dye, which decreased the hydrophilicity of the polymer chain by physical bonding in trans form

122

4.  Photoresponsive Polymers

(Van Der Veen and Prins, 1971). However, the first relevant example of light-responsive polymeric hydrogel was not reported until 1984 (Irie and Kungwatchakun, 1984). The chromophore triphenylmethane leuco was introduced into polyacrylamide or poly(N-isopropylacrylamide) (PNIPAM) hydrogels to obtain a photoregulated swelling and shrinkage due to the reversible light-induced ionization of the chromophore (Irie and Kunwatchakun, 1986; Mamada et al., 1990). Recently, the complexes of azobenzene derivatives and CD have been integrated into hydrogels as responsive engines to light-induced swelling changes. For example, α-CD, a dodecyl-modified poly(acrylic acid) and 4,4′-azodibenzoic acid have been combined to generate a hydrogel with the light-controlled gelsol transition. In the trans form, this system does not form a gel because azobenzene has a great higher affinity with CD than dodecyl and consumes most of CD by forming azobenzene/CD complexes. Under irradiation, the azobenzene derivative undergoes trans-to-cis isomerization; it is released from the α-CD site and allows self-assembly of the dodecyl groups and transition to the gel form (Tomatsu et al., 2005). Azobenzene/ CD-based polymers with different functional groups have been investigated for the reversible sol-gel transition (Wang et al., 2016a; Zhou et al., 2013; Samai et al., 2016). A similar strategy was adapted to dextran hydrogels and applied to control the release of a protein (Fig. 4.21) (Peng et al., 2010). The host-guest molecules, azobenzenyl and β-CD, were attached to the dextran backbone via thiol-ene click reactions. In the trans form, the azobenzenyl group forms the host-guest complex with the β-CD, and this results in effective cross-linking of the dextran and formation of the hydrogel. The green fluorescent protein (GFP) was encapsulated in this system. In the cross-linked system, GFP remains inside the gel, but after UV light irradiation, GFP can diffuse out of the gel and is released. This strategy only works with big molecules (i.e., protein, DNA, or a drug with high molecular weight) that are not able to diffuse outside of the polymer network in the cross-linked form. Azobenzene-containing photoresponsive hydrogels could also be used as models to explore the possibility of applying light to regulate a material's elastic modulus (Rosales et al., 2015). The ability to regulate modulus is attractive for hydrogel materials as it allows for the investigation of the effect of dynamic matrix stiffness on adhered cell behavior. In the study of poly(ethylene glycol)-based hydrogels with azobenzene-­ containing ­cross-linker, reversibly stiffening and softening was achieved by light-­ triggered isomerization. The cis configuration leads to a softening of the hydrogel up to 100-200 Pa (shear storage modulus), and the modulus can recover upon irradiation with visible light. Phototriggered shrinkage has been realized in a PNIPAm hydrogel modified with 1 mol% spirobenzopyran-modified acrylate (Szilagyi et al., 2007). An acidic aqueous solution of this polymer maintained in the dark



4.4 Applications

123

FIG.  4.21  (A) Modification of dextran with azobenzene and CD through the thiol-­ maleimide reaction and (B) schematic representation of phototriggered protein release from the gel. Adapted with permission from Peng, K., Tomatsu, I., Kros, A., 2010. Chem. Commun. 46, 4094–4096.

forms a highly hydrated gel, because most of the spirobenzopyran is present in the positively charged open-ring form. Irradiation with blue light causes the transition to the closed and uncharged form of spirobenzopyran, leading to collapse of the hydrogel in the exposed area. Such shrinkage has been applied to generate a rewritable microrelief (Fig. 4.22). Recently, it was found that changing the spiropyran derivatives ­embedded

124 4.  Photoresponsive Polymers

FIG. 4.22  Left: Chemical structure of a cross-linked PNIPAm hydrogel functionalized with spiropyran and a schematic illustration of the photoinduced shrinking of the hydrogel. Right: (A) Images of the hydrogel layer just after the micropatterned light irradiation. (B) Irradiation times were (red dot) 0, (diamond) 1, and (green rectangle) 3 s. (C) Thickness change of the hydrogel layer in (dot) nonirradiated and (ring) irradiated region (3 s blue light irradiation) vs. time. Adapted with permission from Szilagyi, A., Sumaru, K., Sugiura, S., Takagi, T., Shinbo, T., Zrinyi, M., Kanamori, T., 2007. Chem. Mater. 19, 2730–2732.



4.4 Applications

125

in a PNIPAm hydrogel could induce a significant improvement of the isomerization speed and reversible swelling/shrinking behavior. These spiropyran-based hydrogels could be applied in microfluidic devices (ter Schiphorst et  al., 2015), reversible biomaterials (Wang et  al., 2014b; Sun et al., 2016), actuator designing (Tudor et al., 2016; Delaney et al., 2017), novel stimuli-responsive gel systems (Filipcsei et  al., 2014; Moriyama et al., 2016), and multipurpose microfluidic devices (Stumpel et al., 2014; Ziółkowski et al., 2015). Molecular motors are one class of interesting photoswitches because of their monodirectional rotation. Recently, a light-driven motor has been integrated into hydrogels to induce a macroscopic contraction (Fig. 4.23A) (Li et  al., 2015). The motor was incorporated into polymer networks by connected with four hydrophilic polymer chains. Under UV irradiation, the irreversible monodirectional rotation of the motor can induce continual entanglement and thus macroscopic contraction of the hydrogel. A maximum contraction of nearly 80% was observed by measuring the surface reduction. It is interesting that the gel ruptured after a longer period of irradiation (170 min, Fig. 4.23B). It was attributed to the high tension that can break the double bonds in motor under irradiation (Fig. 4.23C). Phototriggered delivery of Ca2+ cations has been used as a light-induced approach to cross-link alginate hydrogels (Augst et al., 2006; Park and Lee, 2008). Photolabile Ca2+ cages are chelators that change their affinity for Ca2+ upon light exposure from a Kd of several to hundreds nM to several mM (Ellis-Davies, 2008). The affinity change is a consequence of a light-­ induced change in the molecular structure and can be used to change the local concentration of Ca2+ (Mayer and Heckel, 2006; Ellis-Davies, 2008). Cage compound nitr-T has been developed for this purpose and embedded in alginate solution (Cui et al., 2012b). Irradiation at 360 nm released Ca2+ cations, which bond to adjacent α-L-guluronic acid (G) residues of alginate with the chelating interaction of the carboxylic groups (Stokke et  al., 2000). This interaction results in gelation and resulting hydrogel display higher rheological modulus compared to the alginate hydrogel prepared by mixing CaCl2 solution directly (Fig. 4.24) (Cui et al., 2013c). Besides synthetic polymer systems, photoresponsive proteins are also used to prepare photoresponsive hydrogels. One of the examples is based on the combination of tax-interacting protein (TIP-1) and their recombinant protein, arabidopsis thalian protein UVR8. UVR8 is one kind of interesting protein that not only bonds to TIP-1 but also undergoes an ongoing change from homodimer to monomer upon UV irradiation. This unique property allows its derivative UVR8-1 to act as a cross-linker to bond TIP1-based nanofibers. The photoinduced dimer-to-monomer transformation leads to a gel-sol phase transition of the hydrogel of TIP-1-based nanofibers. This material could be applied to protein delivery and cell separation (Zhang et al., 2015b).

126

4.  Photoresponsive Polymers

FIG. 4.23  Macroscopic contraction of hydrogel with light-driven molecular motors. (A) Chemical structures of the motor gels. (B) Snapshots of time-dependent macroscopic contraction of gels upon UV irradiation (0.96 mW cm-2). (C) Overview of macroscopic contraction during irradiation. (D) Small-angle X-ray scattering (SAXS) data obtained before and after irradiation of gels. Adapted with permission from Li, Q., Fuks, G., Moulin, E., Maaloum, M., Rawiso, M., Kulic, I., Foy, J.T., Giuseppone, N., 2015. Nat. Nanotechnol. 10, 161–165.

4.4.4  Photodegradable Materials Polymers with photolabile groups in the main chain undergo chain breakage upon light illumination and can be classified as photodegradable materials. Most of the recent works use the o-nitrobenzyl chromophore, but a few other photopolymerization strategies have also been applied. Silicon-containing polyureas undergo photodegradation upon irradiation at λ >300 nm due to the photoinduced single-electron transfer from the σ C-Si to the adjacent π C = O bond, followed by silyl group migration and solvolysis (Hwu and King, 2005). Biocompatible polyketals and polyacetals have been synthesized and were photolyzed by UV light at 248 nm into carbonyl and hydroxyl product through zwitterionic intermediates and applied for making cell patterns (Pasparakis et  al., 2011). The photolysis requires low energy as a consequence of ionic photo intermediate instead of a radical one. This property makes this system interesting for biomaterial applications as the exposure conditions are compatible with

4.4 Applications

sion from Cui, J., Wang, M., Zheng, Y., Rodríguez Muñiz, G., del Campo, A., 2013. Biomacromolecules 14, 1251–1256.

127

FIG. 4.24  Model of phototriggering alginate hydrogel system with nitr-T(C12)-Ca2+ and its light-induced shrinking. Adapted with permis-

128

4.  Photoresponsive Polymers

living cells. Recently, commercially available o-nitrobenzyl-based photocleavable monomers were added into a polyurethane-based positive photoresist for making micropatterns by photolithography (Garcia-Fernandez et al., 2014). The diblock copolymer of polystyrene (PS) and poly(ethylene oxide) (PEO) with a photodegradable o-nitrobenzyl linker was applied to achieve ordered self-assembly nanostructures (Zhao et al., 2012; Kang and Moon, 2009). The copolymer self-assembled into highly ordered hexagonally packed cylinders oriented perpendicular to the substrate with PS as the continuous phase. UV irradiation cleaves the two blocks by the photolysis reaction of o-nitrobenzyl. The free PEO block was washed with water, which lead to the nanoporous template (Zhao et al., 2011b, 2012; Theato, 2011; Kang and Moon, 2009). Diblock photodegradable copolymers could be obtained by means of two kinds of click reactions and used as a potential for development of drug delivery system and biomaterials (Yamamoto et al., 2016). Polymers with a metal-metal bond (i.e., iron and molybdenum) in main chains of polyesters and polyamides are another kind of photodegradable polymers that are sensitive to visible light. Films of such photodegradable polymers are interesting for agriculture because polymer film degradation can occur with daily light, and the film does not need to be removed (Tyler, 2003). Several applications of nitrobenzenyl derivatives have been recently reported. Fig. 4.25 presents a photodegradable dendron with a hydrophilic and a lipophilic unit connected by the photocleavable group (Yesilyurt et  al., 2011). Micelles of this dendron have been used for encapsulating

FIG. 4.25  Schematic representation of the light-induced disassembly of dendritic micellar assemblies. Adapted with permission from Yesilyurt, V., Ramireddy, R., Thayumanavan, S., 2011. Angew. Chem., Int. Ed. 50, 3038–3042.



4.4 Applications

129

and light-mediated delivering of Nile red. Similarly, different photodegradable polymer-based nanoparticles bearing nitrobenzenyl derivatives have been widely investigated for controlled drug delivery (Yang et al., 2013; Zhang et al., 2015c; Cui et al., 2015; Jalani et al., 2016). Photodegradable hydrogels containing poly(ethylene glycol) (PEG) chains cross-linked with photocleavable nitrobenzyl units have been applied as 3D scaffolds for cell growth with light-tunable mechanical properties (Fig. 4.26) (Kloxin et al., 2009a,b, 2010). Using scanning lasers and two-photon excitation, micrometric resolution of the photodegradation process was possible. 3D channels with reduced cross-linking were created inside the hydrogel to direct cell migration. The elasticity and mechanical properties of the photodegradable materials containing nitrobenzyl units with different functional groups could also be weighed before and after light irradiation (Tibbitt et al., 2013; Yanagawa et al., 2015a; Kharkar et al., 2015). A unique hydrogel-nanoparticle hybrid scaffold containing three distinct components provides a chemically defined, remotely triggerable, and on-demand release of small molecule drugs. Upon photoirradiation, the activation of the phototriggerable compound is designed to initiate a series of intramolecular chemical rearrangements, which would cleave the covalently bound drug and release it from the hydrogel (Shah et al., 2014). Moreover, a targeted, image-guided, and dually locked ­photodegradable

FIG.  4.26  Light-induced disassembly of dendritic micellar assemblies after light exposure. Adapted with permission from Yang, Y., Velmurugan, B., Liu, X., Xing, B., 2013. Small 9, 2937–2944.

130

4.  Photoresponsive Polymers

material could release the anticancer drug and be employed for the ­real-time monitoring of the prodrug and in  vitro cellular imaging by one- and two-photon excitation (Karthik et  al., 2015). Photodegradable hydrogels could also be formed by Michael-type addition reactions and orthogonal click reactions, and the photolithography region can be used to culture cells with high viability and proliferation rates, which can potentially be used to create 3D biomaterials for various tissue-engineering applications (Yanagawa et al., 2015b). Similar photodegradable biomaterials were recently applied for directed cell function and modulating valvular interstitial cell phenotype (Siltanen et al., 2013; Kirschner et al., 2014; Shin et  al., 2014; Yang et  al., 2014b; Arakawa et  al., 2017). Novelty, selective photodegradation on one side of this kind of hydrogel films leads to a class of self-folding structures that can be used for 3D cell culture (Kapyla et  al., 2016). Because of controllable degradation, it is possible to make ­gradient-patterned stiffness (Norris et al., 2016). In addition, nitrobenzyl group was also applied to prepare photodegradable dendrimers (Nazemi and Gillies, 2014; Lai et  al., 2016), miktoarm star polymers (Burts et  al., 2014), and so on (Rajendran et al., 2015; Hwang et al., 2016). In a bioinspired approach, nitrodopamine has been used to end-cap a star PEG and form covalently or metal-cross-linked networks (Fig. 4.27). Upon UV irradiation, the nitrophenylethyl group photolyzed, and the hydrogel degraded. This bioinspired material retains the underwater bonding properties of the mussel (due to the catechol moieties) and incorporates the possibility of light-induced debonding. It represents a new generation of photodegradable biomaterials that can be widely used in biocompatible coating, multiple cell, and medical applications (Shafiq et al., 2012). In addition to nitrobenzenyl, other photocleavable groups such as Irgacure-2959 and azo-motifs have also been applied to prepare photodegradable materials. Irgacure-2959 is a photoinitiator used for photoinduced polymerization. When it is incorporated as a cross-linker in a polymer network, it can act as a photodegradable spacer for controlling a material’s elasticity and swelling ratio (Fig. 4.28A) (Selen et al., 2016). The same strategy also works in azo compounds, which are also well-used radical initiators. One recent example based on poly(vinyl alcohol) with an azo-based cross-linker shows that the UV irradiation causes a degradation and thus triggers a solid-to-liquid phase transition (Fig. 4.28B) (Ayer et al., 2017). Besides the previously referred nitrobenzyl and azo groups, allyl sulfide is another photoresponsive group that can be used to prepare degradable hydrogels. Fig. 4.29 shows the example in which allyl sulfide bis(azide) is used as a photodegradable spacer to connect the PEG network. When the hydrogel is exposed to light in the presence of a photoinitiator, radical species generate and add directly to the allyl sulfide, which induces a radical addition fragmentation chain transfer process. In the case without

4.4 Applications

o-nitrophenyl ethyl moiety. (B) Different strategies used to trigger bonding and debonding upon light exposure of nitrodopamine derivatives. Adapted with permission from Shafiq, Z., Cui, J., Pastor-Pérez, L., Miguel, V.S., Gropeanu, R.A., Serrano, C., del Campo, A., 2012. Angew. Chem. Int. Ed. 124, 4408–4411.

131

FIG.  4.27  Structure of nitrodopamine derivatives and their photocleavage mechanisms. (A) Photolytic reaction of the

132 4.  Photoresponsive Polymers

FIG. 4.28  (A) Photodecomposition of photoinitiator (Irgacue-2959)-based hydrogel under UV irradiation. (B) Formation of azo-cross-linked organogels and light-responsive properties upon UV irradiation. Adapted with permission from Selen, F., Can, V., Temel, G., 2016. RSC Adv. 6, 31692–31697; Ayer, M.A., Schrettl, S., Balog, S., Simon, Y.C., Weder, C., 2007. Soft Matter 13, 4017–4023.



4.4 Applications

133

free thiol inside the system, significant photodegradation could occur due to the direct addition of the photoinitiator radical fragments to the allyl sulfide cross-linker. To increase the efficiency of the photodegradation reaction, free monothiol (mPEG-SH) was added to the system (Fig.  4.29). This radical-initiated thiol-ene exchange reaction made the allyl sulfide hydrogel system degrade. This system can be used for cell encapsulation and release (Brown et al., 2017). When the degradation is normally used to describe a decrease in molecular weight, the solid-to-liquid transition can be included as a specific “degradation” behavior. Fig.  4.30A shows a recent example of light-­ regulated solid-liquid transition (Zhou et al., 2017). The polymer has azobenzene side chains. In the trans state of the chromophore, the polymer has a glass transition temperature (Tg) above room temperature, whereas in the cis state, the polymer shows a Tg below room temperature because of the weak stacking interaction of cis form. Therefore UV light irradiation could induce a solid-to-liquid transition in this polymer at room temperature. Taking advantage of the mobility of the liquid state, this unique transition can be used to prepare smooth surface and self-healing fractures (Fig. 4.30C)

4.4.5  Photoswitchable Liquid Crystalline Elastomers (LCE) for Remote Actuation Photoswitchable units have been incorporated into liquid crystalline polymers. The chromophore is usually part of the mesogenic core, and the light-induced molecular changes directly affect the degree of order of the mesophase and, consequently, its properties such as Curie temperature (Beyer et al., 2007), molecular orientation, symmetry, transition temperatures, etc. (Seki, 2007). One issue of recent interest in photoresponsive liquid crystal elastomers is the possibility to generate photoswitchable actuators. Light exposure induces a change in the conformation of polymer chains from extended to a coiled one, which results in a macroscopic shape change. This actuating principle has been applied for bending LCE films and micropillars (Ohm et al., 2010; Qian et al., 2012). The first example of this kind was reported in 2003 with polymer 28 (Li et al., 2003). A 20-μm thin film of the LCE with the mesogens preferentially oriented in a direction parallel with long axis was obtained. Film contraction in the direction of mesogenic units was observed upon exposure to UV light at 75°C as a consequence of trans-to-cis transition of azobenyl, which i­nduces n ­ ematic-isotropic phase transition. Light-driven flexural-­ torsional response in azobenzene functionalized LCE could be affected by key material parameters with polarized light, complex geometry, boundary conditions, and loading conditions (Smith et al., 2014).

134 4.  Photoresponsive Polymers

FIG. 4.29  Light-triggered radical network degradation in (A) and (B). (C) Incorporation of mPEG-SH allows controlled photodegradation of the gel and tuning of the storage modulus. Adapted with permission from Brown, T.E., Marozas, I.A., Anseth, K.S., 2017. Adv. Mater. 29, 1605001.

4.4 Applications

FIG.  4.30  Photoinduced solid-to-liquid transition of azopolymer-based films. (A) Chemical structure

135

and photoisomerization of azobenzene-containing polymers. (B) Schematic illustration (top) and confocal images (bottom) of surface roughness reduction by photoswtiching of solid-to-liquid transition. (C) Schematic illustration (top) and optical microscopy images (bottom) of scratches healing on a hard azopolymer coatings with different light wavelength. Adapted with permission from Zhou, H., Xue, C., Weis, P., Suzuki, Y., Huang, S., Koynov, K., et al., 2017. Nat. Chem. 9, 145–151.

136

4.  Photoresponsive Polymers

Fig.  4.31 presents a different example where a freestanding LCE film was bent upon irradiation (Ikeda et  al., 2007). Monodomain LCE films with the mesogens aligned in a parallel direction to the surfaces were generated by the method shown in Fig.  4.30A (Yu and Ikeda, 2011). In situ polymerization and cross-linking of the film was carried out in the liquid crystal (LC) phase. In the resulting self-supporting LCE films, the network structure retained the orientation of the mesogens. UV irradiation caused macroscopic bending of the film as a consequence of the contraction induced by the transition of the LC phase into the isotropic phase. Such deformation was reversible by irradiating with alternating UV and visible light sources (Fig. 4.31B). When a polarized light was used, direction-­ controllable bending was achieved in a polydomain LCE films, as shown in Fig. 4.31C (Yu et al., 2003). Recently, similar effects were observed with fibers, which can bend to any shiny direction, like a “sunflower” (Yoshino et  al., 2010). Multifunctional LCEs could be prepared by cross-linked azobenzene chromophores, liquid crystals, and dynamic ester bonds, which exhibits programmable material responses to external stimuli at the molecular level with photomechanical, shape memory, and self-healing properties (Li et al., 2016). Azobenzene moieties could also be modified to silicone elastomer to prepare photodriving LCE materials. The alignment of mesogens and macroscopic shapes can be controlled through the rearrangement of azobenzene groups on the network topology, showing various bending behaviors upon irradiation with UV or visible light (Ube et al., 2016; Ube et al., 2017). When the LCE was embedded with photothermal graphene oxide, a NIR-vis-UV light-controlled actuator can be obtained. The product exhibits excellent processability and mechanical properties (Cheng et  al., 2015). The responsiveness to both UV and visible light was recently applied to get a light-controlled shape-memory effect (Ban et al., 2017). The contraction and bending of LCE have inspired interesting application attempts (Fig. 4.32). An azobenzenyl-containing LCE layer was attached to a flexible polyethylene sheet and used as a photodriven belt (Fig. 4.32A) (Yamada et al., 2008). UV and visible lights were shined simultaneously from different directions and induced a rotation of the belt. This rotation was used to drive a motor device in a counterclockwise direction at room temperature. It represents the first realization of light-driven plastic motors in which light energy was directly converted into mechanically rotational energy. Exploiting bending and stretching effects of a LCE film, bioinspired propulsion was further demonstrated (Fig. 4.32B). The movement was driven by cyclic photoinduced bending and extension (Yamada et al., 2009; Yamada et al., 2009). Recently, molecular relaxation rate after irradiation is taken into ­account for designing novel shape change. Fig.  4.33 shows macroscopic



4.4 Applications

137

FIG. 4.31  Photoresponsive freestanding LCE film. (A) Scheme of the preparation process of freestanding LCE film using an azobenzene-based LC monomer and a cross-linker. (B) Phototriggerred bending mechanism of the monodomain LCE films. (C) Control the bending direction of LCE films by linearly polarized light and its plausible mechanism. Adapted with permission from Yu, H., Ikeda, T., 2011. Adv. Mater. 23, 2149–2180.

­ echanical waves under continual irradiation (Gelebart et  al., 2017). In m this example, molecular tautomerization and structure push-pull strategies were applied to increase the thermal relaxation rate of the azobenzene. The idea was shown by an asymmetric structure with planar alignment on one side but homeotropic alignment on another side. UV irradiation can induce the temperature of the irradiated region to a temperature higher

138

4.  Photoresponsive Polymers

than the Tg of the polymers with a short half-life (1-2s), because of the molecular tautomerization. Phase transition induces a fast contract on the planar alignment sider but an expending on the homeotropic alignment side, which causes a curve downward (planar up) or upward (homeotriopic up). Opposite stress on both sides pushes a crest away (planar up) or toward (homeotriopic up) the light resource and thus results in continuous travelling wave and repeating movement until the light is turned off (Fig. 4.33B and 4.33C). Azobenzenyl-based LCE shows not only the photoswitchable actuating but also macroscopic helical motioning (Tašič et  al., 2013; Lv et  al., 2014; Garcia-Amoros et al., 2014). Inspired by biological systems, a novel spring-like material has been fabricated for converting light energy into mechanical work at the macroscopic scale. It was found that light-­operated molecular-scale motion (cis-trans photoisomerization) can be converted into large macroscopic deformations of the springs. More than a single actuation mode encoded inherently in these chiral objects, the actuation can be reversed when changing from one handedness to the other (Fig. 4.34) (Iamsaard et al., 2014). Another interesting application of a photoresponsive LCE is to make cantilever actuators. To do so, a LCE polymer described by azobenzene monomer and azobenzene cross-linker was cast onto a low-density polyethylene film. The bilayer shows a photomechanical movement, which can drive attached copper coils to cut a magnetic line of force to generate electricity. This simple strategy was claimed to have potential in the applications of the capture and storage of light energy (Tang et al., 2015). Photoresponsive LCE materials are further expended to microfluidic systems recently. The idea is based on the photoinduced asymmetric deformation of tubular microactuators made from azobenzene-based LCE. In the multiple-layer tube, LCE layer would contract under irradiation, which shrinks the tube to offer a driving force to move the droplet in the tube (Fig. 4.35) (Lv et al., 2016). It greatly simplifies microfluidic devices and opens a new way in biomedical and chemical engineering. In addition to permanent covalent cross-linking, dynamic bonding is also able to make LCE. Because of the dynamic nature, these m ­ aterials show interesting properties such as easy-to-reshape, self-healing, fatigue resistance, etc. (Fang et al., 2013). Several interesting examples recently published are based on H-bonding. For example, a novel photodeformable LCE was fabricated by using multivalent hydrogen bonds as ­cross-linkers (Fig. 4.36A). This LCE not only exhibits self-healing properties at low temperature but also presents a reversible photoinduced ­deformation by alternate irradiation of UV and visible light (Fig.  4.36B and C) (Ni et al., 2016). In contrast to the photoinduced isomerization presented in the previous examples, the photothermal heating effect was also applied to get a



4.4 Applications

139

FIG.  4.32  Photoinduced sophisticated 3D motions of a LCE film laminated on a flexible polyethylene sheet. (A) Schematic illustration of a light-driven plastic motor and photographs of time profiles of the rotation. (B) Photographs of photoinduced inchworm walk and the plausible mechanism. A: Adapted with permission from Yamada, M., Kondo, M., Mamiya, J.-i., Yu, Y., Kinoshita, M., Barrett, C.J., Ikeda, T., 2008. Angew. Chem. Int. Ed. 47, 4986–4988; B: Adapted with permission from Yamada, M., Kondo, M., Miyasato, R., Naka, Y., Mamiya, J.-i., Kinoshita, M., Shishido, A., Yu, Y., Barrett, C.J., Ikeda, T., 2009. J. Mater. Chem. 19, 60–62.

­photoinduced deformation. Fig. 4.37 shows a dual-layer, dual-­composition, polysiloxane-based LCE strategy that was developed to mimic the organisms’ complex shape deformations. A NIR absorbing dye (YHD796) was mixed into the azobenzene-based network, and it could induce the LCto-isotropic phase transition because of the photothermal heating ­effect.

140

4.  Photoresponsive Polymers

FIG. 4.33  Waves in a photoactive polymer film. (A) Chemical structures of azo-­derivatives and liquid crystal mesogens. (B) Schematic of the experimental set-up under an oblique-­ incidence light source. (C) Simulation (left) and experimental (right) data for planar-up and homeotropic-up configurations. Adapted with permission from Gelebart, A.H., Mulder, D.J., Varga, M., Konya, A., Vantomme, G., Meijer, E.W., Selinger, R.L.B., Broer, D.J., 2017. Nature 546, 632–636.

To prepare the bilayer, two different kinds of precross-linked layers were fabricated (Fig. 4.37A) and slowly uniaxially stretched to a different angle for orientation. The films could be covalently bonded together to get ­dual-layer LCE that could perform not only bending but also chiral twisting (left-handed and right-handed) under irradiations of different light wavelengths (Fig. 4.37C) (Wang et al., 2016b).

4.5  CONCLUSIONS AND FUTURE TRENDS The future of photoresponsive polymers and smart materials derived from them will depend on the research development in different directions. The development of photoresponsive units acts as an engine of photoresponsive polymer systems. New and more efficient p ­ hotosensitive



141

4.5  Conclusions and Future Trends

dL

UV Visible

1.7

dR dR/dL

1.5 1.3 1.1 0.9 UV (l = 365 nm)

(B)

1

2

3

4

5 6 7 8 Number of cycles

9

10

Magnets

Visible light

UV (l = 365 nm) Magnets

(A)

(C)

FIG.  4.34  Light-driven mechanical device. (A) Deformation under alternate irradiation. (B) Cycles of helical ratio under alternating irradiation. (C) A magnet connected to the kink undergoes a push-pull shuttling motion. Adapted with permission from Iamsaard, S., Asshoff, S.J., Matt, B., Kudernac, T., Cornelissen, J.J., Fletcher, S.P., Katsonis, N., 2014. Nat. Chem. 6, 229–235.

FIG. 4.35  Design and photodeformation of tubular microactuator (TMA). (A) Schematics illustration of the motion of a slug confined in a TMA driven by photodeformation. (B) Lateral photographs of the light-induced motion of a silicone oil slug in a TMA fixed on a substrate. Adapted with permission from Lv, J.A., Liu, Y., Wei, J., Chen, E., Qin, L., Yu, Y., 2016. Nature 537, 179–184.

142

4.  Photoresponsive Polymers

FIG.  4.36  Photoinduced-deformable LCE with dynamic multivalent hydrogen bonds. (A) Schematic and chemical structure of the self-healing photoinduced deformable LCEs. (B) Self-healing process of a fractured fiber. (C) Photoinduced bending and unbending behavior of the healed fiber upon irradiation with UV light at 365 nm and visible light at 470 nm. Adapted with permission from Ni, B., Xie, H., Tang, J., Zhang, H., Chen, E., 2016. Chem. Commun. 52, 10257–10260.

molecular units and switching strategies are required, as well as chromophores sensitive to long wavelengths for compatibility with living organisms and tissues and applications in the biomedical area. The development of polymerization strategies able to incorporate the chromophores at a selected position in complex macromolecular architectures is also an important question, i.e., by mature living control radical polymerization techniques (Hawker et  al., 2001; Kamigaito et  al., 2001; Matyjaszewski and Xia, 2001). Controlled drug delivery will remain as the main application field of photosensitive polymers. Although many systems have been tested in vitro with a dye or a drug, in-vivo applications are scarce and require further development. Close cooperation between organic chemists, polymer chemists, biologists, and medical doctors will be required to push this research to interdisciplinary level (Cosa et al., 2009; Mizukami et al., 2010; Kim et al., 2006; Tanaka et al., 2010; Mal et al., 2003). Lighttriggered actuators will also be an issue for the future, which has been, up to now, mainly based on azobenzene-containing LCEs. However, indirect strategies can also achieve a similar effect and may also extend the irradiation wavelength to broader regions (Torras et al., 2011). All these areas will certainly further grow in the future and expand the application of these systems to unforeseen fields.



4.5  Conclusions and Future Trends

143

FIG.  4.37  Phototunable bending and chiral twisting motion based on a dual-layer, ­ ual-composition, polysiloxane-based LCE. (A) The chemical compositions of the dual-layer d of LCE. (B) Schematic illustration of the preparation protocol of a same-sized-bilayer LCE ribbon material. (C) The bilayer LCE ribbon with a -45 degree angle between the top and bottom layer was irradiated under 365 nm UV light and an 808 nm NIR light, respectively. Adapted with permission from Wang, M., Lin, B., Yang, H., 2016. Nat. Commun. 7, 13981.

144

4.  Photoresponsive Polymers

References Adhikari, B., Yamada, Y., Yamauchi, M., Wakita, K., Lin, X., Aratsu, K., Ohba, T., Karatsu, T., Hollamby, M.J., Shimizu, N., Takagi, H., Haruki, R., Adachi, S.I., Yagai, S., 2017. Nat. Commun. 8, 15254. Akiyama, H., Tamaoki, N., 2007. Macromolecules 40, 5129–5132. Alicante, R., Cases, R., Villacampa, B., Blasco, E., 2012. Macromol. Chem. Phys. 213, 776–783. Al-Malaika, S., Hewitt, C., Sheena, H.H., 2010. Photodegradable Polymers. John Wiley & Sons, Inc.603–626. Anastasiadis, S.H., Lygeraki, M.I., Athanassiou, A., Farsari, M., Pisignano, D., 2008. J. Adhes. Sci. Technol. 22, 1853–1868. Andersson, J., Li, S., Lincoln, P., Andréasson, J., 2008. J. Am. Chem. Soc. 130, 11836–11837. Angiolini, L., Benelli, T., Giorgini, L., Golemme, A., Mazzocchetti, L., Termine, R., 2014. Dyes Pigments 102, 53–62. Arakawa, C.K., Badeau, B.A., Zheng, Y., DeForest, C.A., 2017. Adv. Mater. 29. Areephong, J., Kudernac, T., de Jong, J.J.D., Carroll, G.T., Pantorott, D., Hjelm, J., Browne, W.R., Feringa, B.L., 2008. J. Am. Chem. Soc. 130, 12850–12851. Arumugam, S., Popik, V.V., 2009. J. Am. Chem. Soc. 131, 11892–11899. Arumugam, S., Popik, V.V., 2011a. J. Am. Chem. Soc. 133, 5573–5579. Arumugam, S., Popik, V.V., 2011b. J. Am. Chem. Soc. 133, 15730–15736. Arumugam, S., Orski, S.V., Locklin, J., Popik, V.V., 2012. J. Am. Chem. Soc. 134, 179–182. Augst, A.D., Kong, H.J., Mooney, D.J., 2006. Macromol. Biosci. 6, 623–633. Ayer, M.A., Schrettl, S., Balog, S., Simon, Y.C., Weder, C., 2017. Soft Matter 13, 4017–4023. Babin, J., Lepage, M., Zhao, Y., 2008. Macromolecules 41, 126–1253. Babin, J., Pelletier, M., Lepage, M., Allard, J.-F., Morris, D., Zhao, Y., 2009. Angew. Chem. Int. Ed. 48, 3329–3332. Ban, J., Mu, L., Yang, J., Chen, S., Zhuo, H., 2017. J. Mater. Chem. A 5, 14514–14518. Bansal, A., Zhang, Y., 2014. Acc. Chem. Res. 47, 3052–3060. Barille, R., Ahmadi-Kandjani, S., Ortyl, E., Kucharski, S., Nunzi, J.-M., 2009. New J. Chem. 33, 1207–1210. Barille, R., Janik, R., Kucharski, S., Eyer, J., Letournel, F., 2011. Colloids Surf. B 88, 63–71. Barman, S., Mukhopadhyay, S.K., Gangopadhyay, M., Biswas, S., Dey, S., Singh, N.D.P., 2015. J. Mater. Chem. B 3, 3490–3497. Barrett, C.J., Mamiya, J.-i., Yager, K.G., Ikeda, T., 2007. Soft Matter 3, 1249–1261. Berda, E.B., Foster, E.J., Meijer, E.W., 2010. Macromolecules 43, 1430–1437. Bertarelli, C., Bianco, A., Boffa, V., Mirenda, M., Gallazzi, M.C., Zerbi, G., 2004. Adv. Funct. Mater. 14, 1129–1133. Bertrand, O., Gohy, J.-F., Fustin, C.-A., 2011. Polym. Chem. 2, 2284–2292. Bertrand, O., Fustin, C.-A., Gohy, J.-F., 2012. ACS Macro Lett. 1, 949–953. Beyer, P., Krueger, M., Giesselmann, F., Zentel, R., 2007. Adv. Funct. Mater. 17, 109–114. Bisoyi, H.K., Li, Q., 2014. Acc. Chem. Res. 47, 3184–3195. Bisoyi, H.K., Li, Q., 2016. Chem. Rev. 116, 15089–15166. Blasco, E., Barrio, J.d., Sánchez-Somolinos, C., Piñol, M., Oriol, L., 2013a. Polym. Chem. 4, 2246. Blasco, E., Serrano, J.L., Piñol, M., Oriol, L., 2013b. Macromolecules 46, 5951–5960. Bobrovsky, A., Mochalov, K., Chistyakov, A., Oleinikov, V., Shibaev, V., 2014. J. Photochem. Photobiol. A Chem. 275, 30–36. Bobrovsky, A., Shibaev, V., Piryazev, A., Anokhin, D.V., Ivanov, D.A., Sinitsyna, O., Hamplova, V., Kaspar, M., Bubnov, A., 2017. Macromol. Chem. Phys. 218, 1700127. Bradshaw, D.S., Andrews, D.L., 2011. Polymer 3, 2053–2077. Brown, A.A., Azzaroni, O., Huck, W.T.S., 2009a. Langmuir 25, 1744–1749. Brown, A.A., Azzaroni, O., Fidalgo, L.M., Huck, W.T.S., 2009b. Soft Matter 5, 2738–2745. Brown, T.E., Marozas, I.A., Anseth, K.S., 2017. Adv. Mater. 29, 1605001.

REFERENCES 145

Brunsen, A., Cui, J., Ceolin, M., del, a.A., Soler-Illia, G.J.A.A., Azzaroni, O., 2012. Chem. Commun. 48, 1422–1424. Burts, A.O., Gao, A.X., Johnson, J.A., 2014. Macromol. Rapid Commun. 35, 168–173. Bushuyev, O.S., Aizawa, M., Shishido, A., Barrett, C.J., 2017. Macromol. Rapid Commun. Byrne, R., Ventura, C., Benito, L.F., Walther, A., Heise, A., Diamond, D., 2011. Biosens. Bioelectron. 26, 1392–1398. Cano-Marín, A.R., Díez-Barra, E., Rodríguez-López, J., 2005. Tetrahedron 61, 395–400. Cao, P.-F., Su, Z., de Leon, A., Advincula, R.C., 2015. ACS Macro Lett. 4, 58–62. Cardenas-Daw, C., Kroeger, A., Schaertl, W., Froimowicz, P., Landfester, K., 2012. Macromol. Chem. Phys. 213, 144–156. Chamberlayne, C.F., Lepekhina, E.A., Saar, B.D., Peth, K.A., Walk, J.T., Harbron, E.J., 2014. Langmuir 30, 14658–14669. Chaterji, S., Kwon, I.K., Park, K., 2007. Prog. Polym. Sci. 32, 1083–1122. Chen, C., Liu, G., Liu, X., Pang, S., Zhu, C., Lv, L., Ji, J., 2011. Polym. Chem. 2, 1389–1397. Chen, C.-J., Liu, G.-Y., Liu, X.-S., Li, D.-D., Ji, J., 2012a. New J. Chem. 36, 694–701. Chen, C.-J., Jin, Q., Liu, G.-Y., Li, D.-D., Wang, J.-L., Ji, J., 2012b. Polymer 53, 3695–3703. Cheng, Z., Wang, T., Li, X., Zhang, Y., Yu, H., 2015. ACS Appl. Mater. Interfaces 7, 27494–27501. Cheng, R., Chen, J., Liu, Z., Liu, Z., Jiang, J., 2016. Macromol. Rapid Commun. 37, 514–520. Chi, X., Ji, X., Xia, D., Huang, F., 2015. J. Am. Chem. Soc. 137, 1440–1443. Cho, H.J., Chung, M., Shim, M.S., 2015. J. Ind. Eng. Chem. 31, 15–25. Choi, H., Lee, H., Kang, Y., Kim, E., Kang, S.O., Ko, J., 2005. J. Organomet. Chem. 70, 8291–8297. Concellón, A., Blasco, E., Martínez-Felipe, A., Martínez, J.C., Šics, I., Ezquerra, T.A., Nogales, A., Piñol, M., Oriol, L., 2016. Macromolecules 49, 7825–7836. Cosa, G., Lukeman, M., Scaiano, J.C., 2009. Acc. Chem. Res. 42, 599–607. Coumes, F., Malfait, A., Bria, M., Lyskawa, J., Woisel, P., Fournier, D., 2016a. Polym. Chem. 7, 4682–4692. Coumes, F., Woisel, P., Fournier, D., 2016b. Macromolecules 49, 8925–8932. Cozan, V., Hulubei, C., Airinei, A., Morariu, S., 2016. RSC Adv. 6, 101900–101910. Cui, J., del Campo, A., 2012. Chem. Commun. 48, 9302–9304. Cui, J., Azzaroni, O., del Campo, A., 2011. Macromol. Rapid Commun. 32, 1699–1703. Cui, J., Huong, N.T., Ceolín, M., Berger, R.d., Azzaroni, O., del Campo, A., 2012a. Macromolecules 45, 3213–3220. Cui, J., Gropeanu, R.A., Stevens, D.R., Rettig, J., del Campo, A., 2012b. J. Am. Chem. Soc. 134, 7733–7740. Cui, J., San Miguel, V., del Campo, A., 2013a. Macromol. Rapid Commun. 34, 310–329. Cui, J., Wang, D., Koynov, K., del Campo, A., 2013b. ChemPhysChem 14, 2932–2938. Cui, J., Wang, M., Zheng, Y., Rodríguez Muñiz, G., del Campo, A., 2013c. Biomacromolecules 14, 1251–1256. Cui, L., Zhang, F., Wang, Q., Lin, H., Yang, C., Zhang, T., Tong, R., An, N., Qu, F., 2015. J. Mater. Chem. B 3, 7046–7054. de Gracia Lux, C., McFearin, C.L., Joshi-Barr, S., Sankaranarayanan, J., Fomina, N., Almutairi, A., 2012. ACS Macro Lett. 1, 922–926. De Greef, T.F.A., Smulders, M.M.J., Wolffs, M., Schenning, A.P.H.J., Sijbesma, R.P., Meijer, E.W., 2009. Chem. Rev. 109, 5687–5754. De, S.F.B., Guerreiro, J.D.T., Ma, M., Anderson, D.G., Drum, C.L., Sinisterra, R.D., Langer, R., 2010. J. Mater. Chem. 20, 9910–9917. Delaney, C., McCluskey, P., Coleman, S., Whyte, J., Kent, N., Diamond, D., 2017. Lab Chip 17, 2013–2021. Deloncle, R., Caminade, A.-M., 2010. J. Photochem. Photobiol. C 11, 25–45. Ding, J., Liu, G., 1998. Macromolecules 31, 6554–6558. Ding, T., Yang, W., Luo, Z., Liu, J., Zhang, J., Cai, K., 2017. Mater. Lett. 209, 392–395. Dinu, I.A., Duskey, J.T., Car, A., Palivan, C.G., Meier, W., 2016. Polym. Chem. 7, 3451–3464.

146

4.  Photoresponsive Polymers

Dong, R., Liu, Y., Zhou, Y., Yan, D., Zhu, X., 2011. Polym. Chem. 2, 2771–2774. Draper, E.R., Adams, D.J., 2016. Chem. Commun. 52, 8196–8206. Ellis-Davies, G.C.R., 2008. Chem. Rev. 108, 1603–1613. Endo, M., Fukui, T., Jung, S.H., Yagai, S., Takeuchi, M., Sugiyasu, K., 2016. J. Am. Chem. Soc. 138, 14347–14353. Ercole, F., Davis, T.P., Evans, R.A., 2010. Polym. Chem. 1, 37–54. Everlof, G.J., Jaycox, G.D., 2000. Polymer 41, 6527–6536. Fang, L., Zhang, H., Li, Z., Zhang, Y., Zhang, Y., Zhang, H., 2013. Macromolecules 46, 7650–7660. Fang, J.-Y., Lin, Y.-K., Wang, S.-W., Li, Y.-C., Lee, R.-S., 2015. React. Funct. Polym. 95, 46–54. Feringa, B.L., van Delden, R.A., Koumura, N., Geertsema, E.M., 2000. Chem. Rev. 100, 1789–1816. Filipcsei, G., Sumaru, K., Takagi, T., Kanamori, T., Zrinyi, M., 2014. J. Mol. Liq. 189, 63–67. Folmer, B.J.B., Cavini, E., Sijbesma, R.P., Meijer, E.W., 1998. Chem. Commun. 1847–1848. Foster, E.J., Berda, E.B., Meijer, E.W., 2009. J. Am. Chem. Soc. 131, 6964–6966. Frey, M.T., Wang, Y.-l., 2009. Soft Matter 5, 1918–1924. Fries, K.H., Driskell, J.D., Samanta, S., Locklin, J., 2010. Anal. Chem. 82, 3306–3314. Fu, S., Zhao, Y., 2015. Macromolecules 48, 5088–5098. Gao, Z., Han, Y., Chen, S., Li, Z., Tong, H., Wang, F., 2017. ACS Macro Lett. 6, 541–545. Garcia-Amoros, J., Martinez, M., Finkelmann, H., Velasco, D., 2014. Phys. Chem. Chem. Phys. 16, 8448–8454. Garcia-Fernandez, L., Specht, A., del Campo, A., 2014. Macromol. Rapid Commun. 35, 1801–1807. Gelebart, A.H., Mulder, D.J., Varga, M., Konya, A., Vantomme, G., Meijer, E.W., Selinger, R.L.B., Broer, D.J., 2017. Nature 546, 632–636. Goeldner, M., Givens, R., 2005. Dynamic Studies in Biology: Phototriggers, Photoswitches and Caged Biomolecules. Wiley, VCH, Weinheim159. Gohy, J.F., Zhao, Y., 2013. Chem. Soc. Rev. 42, 7117–7129. Gong, Y.-H., Li, C., Yang, J., Wang, H.-Y., Zhuo, R.-X., Zhang, X.-Z., 2011. Macromolecules 44, 7499–7502. Goodwin, A.P., Mynar, J.L., Ma, Y., Fleming, G.R., Frechet, J.M.J., 2005. J. Am. Chem. Soc. 127, 9952–9953. Goulet-Hanssens, A., Barrett, C.J., 2013. J. Polym. Sci., Part A: Polym. Chem. 51, 3058–3070. Green, M.M., Park, J.-W., Sato, T., Teramoto, A., Lifson, S., Selinger, R.L.B., Selinger, J.V., 1999. Angew. Chem. Int. Ed. 38, 3138–3154. Green, M.D., Foster, A.A., Greco, C.T., Roy, R., Lehr, R.M., Epps 3rd, T.H., Sullivan, M.O., 2014. Polym. Chem. 5, 5535–5541. Gupta, S., Uhlmann, P., Agrawal, M., Chapuis, S., Oertel, U., Stamm, M., 2008. Macromolecules 41, 2874–2879. Gupta, M.K., Balikov, D.A., Lee, Y., Ko, E., Yu, C., Chun, Y.W., Sawyer, D.B., Kim, W.S., Sung, H.-J., 2017. J. Mater. Chem. B 5, 5206–5217. Han, D., Tong, X., Zhao, Y., 2012. Langmuir 28, 2327–2331. Hawker, C.J., Bosman, A.W., Harth, E., 2001. Chem. Rev. 101, 3661–3688. Hayakawa, J., Momotake, A., Arai, T., 2003. Chem. Commun. 94–95. He, J., Zhao, Y., 2011. Dyes Pigments 89, 278–283. He, D., Arisaka, Y., Masuda, K., Yamamoto, M., Takeda, N., 2017. Acta Biomater. 51, 101–111. Hidayat, R., Nishihara, Y., Fujii, A., Ozaki, M., Yoshino, K., Frankevich, E., 2002. Phys. Rev. B: Condens. Matter Mater. Phys. 66, 075214/075211–075214/075217. Hu, X., Tian, J., Liu, T., Zhang, G., Liu, S., 2013. Macromolecules 46, 6243–6256. Huang, Y., Dong, R., Zhu, X., Yan, D., 2014. Soft Matter 10, 6121–6138. Hwang, L., Guardado-Alvarez, T.M., Ayaz-Gunner, S., Ge, Y., Jin, S., 2016. Langmuir 32, 3963–3969. Hwu, J.R., King, K.Y., 2005. Chem. Eur. J. 3805–3815.

REFERENCES 147

Iamsaard, S., Asshoff, S.J., Matt, B., Kudernac, T., Cornelissen, J.J., Fletcher, S.P., Katsonis, N., 2014. Nat. Chem. 6, 229–235. Ikeda, T., Mamiya, J.-i., Yu, Y., 2007. Angew. Chem. Int. Ed. 46, 506–528. Inomata, K., Kawasaki, S., Kameyama, A., Nishikubo, T., 2000. React. Funct. Polym. 45, 1–9. Iqbal, D., Samiullah, M.H., 2013. Materials (Basel) 6, 116–142. Irie, M., 1993. Photo- and Chemical-Responsive Polymer Solution and Gel Systems, Vol. B. Elsevier159–164. Irie, M., Ikeda, T., 1997. Photo-Responsive Polymers. Dekker65–116. Irie, M., Kungwatchakun, D., 1984. Makromol. Chem., Rapid Commun. 5, 829–832. Irie, M., Kunwatchakun, D., 1986. Macromolecules 19, 2476–2480. Jalani, G., Naccache, R., Rosenzweig, D.H., Haglund, L., Vetrone, F., Cerruti, M., 2016. J. Am. Chem. Soc. 138, 1078–1083. Jeon, I., Cui, J., Illeperuma, W.R.K., Aizenberg, J., Vlassak, J.J., 2016. Adv. Mater. 28, 4678–4683. Ji, W., Li, N., Chen, D., Qi, X., Sha, W., Jiao, Y., Xu, Q., Lu, J., 2013. J. Mater. Chem. B 1, 5942. Jiang, D.L., Aida, T., 1997. Nature 388, 454–456. Jiang, J., Tong, X., Zhao, Y., 2005. J. Am. Chem. Soc. 127, 8290–8291. Jiang, J., Tong, X., Morris, D., Zhao, Y., 2006. Macromolecules 39, 4633–4640. Jiang, J., Qi, B., Lepage, M., Zhao, Y., 2007. Macromolecules 40, 790–792. Jiang, Z., Li, H., You, Y., Wu, X., Shao, S., Gu, Q., 2015. J. Biomed. Mater. Res. A 103, 65–70. Jochum, F.D., zur, B.L., Roth, P.J., Theato, P., 2009. Macromolecules 42, 7854–7862. Jones, C.L., Tansell, A.J., Easun, T.L., 2016. J. Mater. Chem. A 4, 6714–6723. Kalva, N., Parekh, N., Ambade, A.V., 2015. Polym. Chem. 6, 6826–6835. Kamigaito, M., Ando, T., Sawamoto, M., 2001. Chem. Rev. 101, 3689–3746. Kaner, P., Hu, X., Thomas 3rd, S.W., Asatekin, A., 2017. ACS Appl. Mater. Interfaces 9, 13619–13631. Kang, M., Moon, B., 2009. Macromolecules 42, 455–458. Kapyla, E., Delgado, S.M., Kasko, A.M., 2016. ACS Appl. Mater. Interfaces 8, 17885–17893. Karthik, S., Prashanth Kumar, B.N., Gangopadhyay, M., Mandal, M., Singh, N.D.P., 2015. J. Mater. Chem. B 3, 728–732. Kawai, T., Kunitake, T., Irie, M., 1999. Chem. Lett. 905–906. Kawai, T., Nakashima, Y., Irie, M., 2005. Adv. Mater. 17, 309–314. Kevwitch, R., McGrath, D.V., 2001. Polym. Mater. Sci. Eng. 84, 749–750. Kevwitch, R.M., McGrath, D.V., 2002. Synthesis 1171–1176. Kevwitch, R.M., McGrath, D.V., 2007. New J. Chem. 31, 1332–1336. Keyvan Rad, J., Mahdavian, A.R., Salehi-Mobarakeh, H., Abdollahi, A., 2016. Macromolecules 49, 141–152. Kharkar, P.M., Kiick, K.L., Kloxin, A.M., 2015. Polym. Chem. 6, 5565–5574. Kim, E., Lee, H.W., 2006. J. Mater. Chem. 16, 1384–1389. Kim, E., Cho, S.Y., Ahn, K.-H., 2005. Mol. Cryst. Liq. Cryst. 430, 135–145. Kim, H.-C., Hartner, S., Behe, M., Behr, T.M., Hampp, N.A., 2006. J. Biomed. Opt. 11, 034024/034021–034024/034029. Kim, J., Hanna, J.A., Byun, M., Santangelo, C.D., Hayward, R.C., 2012. Science 335, 1201–1205. Kim, S., Inoue, W., Hirano, S., Yagi, R., Kuwahara, Y., Ogata, T., Kurihara, S., 2014a. Polymer 55, 871–877. Kim, D.Y., Lee, S.A., Choi, Y.J., Hwang, S.H., Kuo, S.W., Nah, C., Lee, M.H., Jeong, K.U., 2014b. Chemistry 20, 5689–5695. Kirschner, C.M., Alge, D.L., Gould, S.T., Anseth, K.S., 2014. Adv. Healthc. Mater. 3, 649–657. Kloxin, A.M., Kasko, A.M., Salinas, C.N., Anseth, K.S., 2009a. Science 324, 59–63. Kloxin, A.M., Benton, J.A., Anseth, K.S., 2009b. Biomaterials 31, 1–8. Kloxin, A.M., Tibbitt, M.W., Kasko, A.M., Fairbairn, J.A., Anseth, K.S., 2010. Adv. Mater. 22, 61–66. Kong, L., Wong, H.L., Tam, A.Y., Lam, W.H., Wu, L., Yam, V.W., 2014. ACS Appl. Mater. Interfaces 6, 1550–1562.

148

4.  Photoresponsive Polymers

Koskela, J.E., Liljestrom, V., Lim, J., Simanek, E.E., Ras, R.H., Priimagi, A., Kostiainen, M.A., 2014. J. Am. Chem. Soc. 136, 6850–6853. Koyama, T., Hatano, K., Matsuoka, K., Esumi, Y., Terunuma, D., 2009. Molecules 14, 2226–2234. Kozanecka-Szmigiel, A., Szmigiel, D., Switkowski, K., Schab-Balcerzak, E., Siwy, M., 2011. Opt. Appl. 41, 777–785. Krauss, U., Lee, J., Benkovic, S.J., Jaeger, K.-E., 2010. Microb. Biotechnol. 3, 15–23. Kravchenko, A., Shevchenko, A., Ovchinnikov, V., Priimagi, A., Kaivola, M., 2011. Adv. Mater. 23, 4174–4177. Kuad, P., Miyawaki, A., Takashima, Y., Yamaguchi, H., Harada, A., 2007. J. Am. Chem. Soc. 129, 12630–12631. Lai, C.-T., Hong, J.-L., 2010. J. Phys. Chem. B 114, 10302–10310. Lai, Y.-S., Kao, C.-L., Chen, Y.-P., Fang, C.-C., Hu, C.-C., Chu, C.-C., 2016. New J. Chem. 40, 2601–2608. Landry, M.J., Applegate, M.B., Bushuyev, O.S., Omenetto, F.G., Kaplan, D.L., Cronin-Golomb, M., Barrett, C.J., 2017. Soft Matter 13, 2903–2906. Lee, S., Flood, A.H., 2013. J. Phys. Org. Chem. 26, 79–86. Lee, H.-i., Wu, W., Oh, J.K., Mueller, L., Sherwood, G., Peteanu, L., Kowalewski, T., Matyjaszewski, K., 2007. Angew. Chem. Int. Ed. 46, 2453–2457. Lee, S.Y., Lee, H., In, I., Park, S.Y., 2014. Eur. Polym. J. 57, 1–10. Li, M.H., Keller, P., Li, B., Wang, X., Brunet, M., 2003. Adv. Mater. 15, 569–572. Li, Y., Jia, X., Gao, M., He, H., Kuang, G., Wei, Y., 2010. J. Polym. Sci. Part A: Polym. Chem. 48, 551–557. Li, M., Yan, H., Teh, C., Korzh, V., Zhao, Y., 2014a. Chem. Commun. 50, 9745–9748. Li, L., Deng, X.-X., Li, Z.-L., Du, F.-S., Li, Z.-C., 2014b. Macromolecules 47, 4660–4667. Li, Q., Fuks, G., Moulin, E., Maaloum, M., Rawiso, M., Kulic, I., Foy, J.T., Giuseppone, N., 2015. Nat. Nanotechnol. 10, 161–165. Li, Y., Rios, O., Keum, J.K., Chen, J., Kessler, M.R., 2016. ACS Appl. Mater. Interfaces 8, 15750–15757. Liao, X., Chen, G., Liu, X., Chen, W., Chen, F., Jiang, M., 2010. Angew. Chem. Int. Ed. 49, 4409–4413. Linsley, C.S., Wu, B.M., 2017. Ther. Deliv. 8, 89–107. Liu, G., Dong, C.M., 2012. Biomacromolecules 13, 1573–1583. Liu, J., Qiao, Y., Liu, Z., Wang, L., 2014a. RSC Adv. 4, 21093–21100. Liu, L., Rui, L., Gao, Y., Zhang, W., 2014b. Polym. Chem. 5, 5453. Liu, Y., Li, G., Chen, J., Liu, Z., Liu, Z., Jiang, J., 2017. Macromol. Rapid Commun. 38. Lomadze, N., Kopyshev, A., Rühe, J.r., Santer, S., 2011. Macromolecules 44, 7372–7377. Lovrien, R., 1967. Proc. Natl. Acad. Sci. U. S. A. 57, 236–242. Luo, Q., Cheng, H., Tian, H., 2011. Polym. Chem. 2, 2435–2443. Lv, J.A., Wang, W., Xu, J., Ikeda, T., Yu, Y., 2014. Macromol. Rapid Commun. 35, 1266–1272. Lv, J.A., Liu, Y., Wei, J., Chen, E., Qin, L., Yu, Y., 2016. Nature 537, 179–184. Lyon, L.A., Meng, Z., Singh, N., Sorrell Courtney, D., St John, A., 2009. Chem. Soc. Rev. 38, 865–874. Mal, N.K., Fujiwara, M., Tanaka, Y., 2003. Nature 421, 350–353. Mamada, A., Tanaka, T., Kungwatchakun, D., Irie, M., 1990. Macromolecules 23, 1517–1519. Mandal, A.K., Gangopadhyay, M., Das, A., 2015. Chem. Soc. Rev. 44, 663–676. Marsella, M.J., Wang, Z.-Q., Mitchell, R.H., 2000. Org. Lett. 2, 2979–2982. Matsumoto, S., Yamaguchi, S., Ueno, S., Komatsu, H., Ikeda, M., Ishizuka, K., Iko, Y., Tabata, K.V., Aoki, H., Ito, S., Noji, H., Hamachi, I., 2008. Chem. Eur. J. 14, 3977–3986. Matyjaszewski, K., Xia, J., 2001. Chem. Rev. 101, 2921–2990. Maxein, G., Zentel, R., 1995. Macromolecules 28, 8438–8440. Mayer, G., Heckel, A., 2006. Angew. Chem. Int. Ed. 45, 4900–4921. Mayer, S., Zentel, R., 1998. Macromol. Chem. Phys. 199, 1675–1682.

REFERENCES 149

Mazzier, D., Maran, M., Polo Perucchin, O., Crisma, M., Zerbetto, M., Causin, V., Toniolo, C., Moretto, A., 2014. Macromolecules 47, 7272–7283. Mizukami, S., Hosoda, M., Satake, T., Okada, S., Hori, Y., Furuta, T., Kikuchi, K., 2010. J. Am. Chem. Soc. 132, 9524–9525. Moriyama, K., Sumaru, K., Takagi, T., Satoh, T., Kanamori, T., 2016. RSC Adv. 6, 44212–44215. Mukhopadhyay, R.D., Praveen, V.K., Ajayaghosh, A., 2014. Mater. Horiz. 1, 572–576. Nachtigall, O., Kordel, C., Urner, L.H., Haag, R., 2014. Angew. Chem. Int. Ed. 53, 9669–9673. Nakahama, T., Kitagawa, D., Sotome, H., Ito, S., Miyasaka, H., Kobatake, S., 2017. J. Phys. Chem. C 121, 6272–6281. Nayak, A., Liu, H., Belfort, G., 2006. Angew. Chem. Int. Ed. 45, 4094–4098. Nazemi, A., Gillies, E.R., 2014. Chem. Commun. (Camb.) 50, 11122–11125. Nguyen, T.-T.-T., Turp, D., Wang, D., Nolscher, B., Laquai, F., Mullen, K., 2011. J. Am. Chem. Soc. 133, 11194–11204. Ni, B., Xie, H., Tang, J., Zhang, H., Chen, E., 2016. Chem. Commun. 52, 10257–10260. Nor, I., Enea, R., Hurduc, V., Bercea, M., 2007. J. Optoelectron. Adv. Mater. 9, 3639–3644. Nori, Y., Chu, C., 1983. Can. J. Chem. 61, 300–305. Norris, S.C.P., Tseng, P., Kasko, A.M., 2016. ACS Biomater. Sci. Eng. 2, 1309–1318. Ogoshi, T., Yoshikoshi, K., Aoki, T., Yamagishi, T.A., 2013. Chem. Commun. 49, 8785–8787. Ohm, C., Brehmer, M., Zentel, R., 2010. Adv. Mater. 22, 3366–3387. Olejniczak, J., Chan, M., Almutairi, A., 2015. Macromolecules 48, 3166–3172. Pandey, S., Kolli, B., Mishra, S.P., Samui, A.B., 2012. J. Polym. Sci., Part A: Polym. Chem. 50, 1205–1215. Paramonov, S.V., Lokshin, V., Fedorova, O.A., 2011. J. Photochem. Photobiol. C 12, 209–236. Park, H., Lee, K.Y., 2008. Nat. Polym. Biomed. Appl. 515–532. Pasparakis, G., Manouras, T., Selimis, A., Vamvakaki, M., Argitis, P., 2011. Angew. Chem. Int. Ed. 50, 4142–4145. Pasparakis, G., Manouras, T., Argitis, P., Vamvakaki, M., 2012. Macromol. Rapid Commun. 33, 183–198. Peng, K., Tomatsu, I., Kros, A., 2010. Chem. Commun. 46, 4094–4096. Pijper, D., Jongejan, M.G.M., Meetsma, A., Feringa, B.L., 2008. J. Am. Chem. Soc. 130, 4541–4552. Price, J.T., Ragogna, P.J., 2013. Chemistry 19, 8473–8477. Priimagi, A., Barrett, C.J., Shishido, A., 2014. J. Mater. Chem. C 2, 7155–7162. Qian, Q., Chen, J., Li, M.-H., Keller, P., He, D., 2012. J. Mater. Chem. 22, 4669–4674. Qu, D.H., Wang, Q.C., Zhang, Q.W., Ma, X., Tian, H., 2015. Chem. Rev. 115, 7543–7588. Radl, S.V., Schipfer, C., Kaiser, S., Moser, A., Kaynak, B., Kern, W., Schlögl, S., 2017. Polym. Chem. 8, 1562–1572. Rajendran, S., Raghunathan, R., Hevus, I., Krishnan, R., Ugrinov, A., Sibi, M.P., Webster, D.C., Sivaguru, J., 2015. Angew. Chem. Int. Ed. 54, 1159–1163. Rianna, C., Calabuig, A., Ventre, M., Cavalli, S., Pagliarulo, V., Grilli, S., Ferraro, P., Netti, P.A., 2015. ACS Appl. Mater. Interfaces 7, 16984–16991. Rocha, L., Păiuş, C.-M., Luca-Raicu, A., Resmerita, E., Rusu, A., Moleavin, I.-A., Hamel, M., Branza-Nichita, N., Hurduc, N., 2014. J. Photochem. Photobiol. A Chem. 291, 16–25. Rosales, A.M., Mabry, K.M., Nehls, E.M., Anseth, K.S., 2015. Biomacromolecules 16, 798–806. Roth, P.J., Jochum, F.D., Forst, F.R., Zentel, R., Theato, P., 2010. Macromolecules 43, 4638–4645. Royes, J., Rebole, J., Custardoy, L., Gimeno, N., Oriol, L., Tejedor, R.M., Pinol, M., 2012. J. Polym. Sci., Part A: Polym. Chem. 50, 1579–1590. Ruchmann, J., Sebai, S.C., Tribet, C., 2011. Macromolecules 44, 604–611. Rwei, A.Y., Wang, W., Kohane, D.S., 2015. Nano Today 10, 451–467. Ryabchun, A., Sakhno, O., Stumpe, J., Bobrovsky, A., 2017. Adv. Opt. Mater. 5, 1700314. Sajti, S., Kerekes, A., Ramanujam, P.S., Lorincz, E., 2002. Appl. Phys. B Lasers Opt. 75, 677–685. Samai, S., Sapsanis, C., Patil, S.P., Ezzeddine, A., Moosa, B.A., Omran, H., Emwas, A.H., Salama, K.N., Khashab, N.M., 2016. Soft Matter 12, 2842–2845.

150

4.  Photoresponsive Polymers

San Jose, B.A., Ashibe, T., Tada, N., Yorozuya, S., Akagi, K., 2014. Adv. Funct. Mater. 24, 6166–6171. Schab-Balcerzak, E., Grucela-Zajac, M., Kozanecka-Szmigiel, A., Switkowski, K., 2012. Opt. Mater. 34, 733–740. Schumers, J.-M., Fustin, C.-A., Gohy, J.-F., 2010a. Macromol. Rapid Commun. 31, 1588–1607. Schumers, J.-M., Gohy, J.-F., Fustin, C.-A., 2010b. Polym. Chem. 1, 161–163. Schumers, J.-M., Bertrand, O., Fustin, C.-A., Gohy, J.-F., 2012. J. Polym. Sci., Part A: Polym. Chem. 50, 599–608. Seki, T., 2007. Bull. Chem. Soc. Jpn. 80, 2084–2109. Selen, F., Can, V., Temel, G., 2016. RSC Adv. 6, 31692–31697. Shafiq, Z., Cui, J., Pastor-Pérez, L., Miguel, V.S., Gropeanu, R.A., Serrano, C., del Campo, A., 2012. Angew. Chem. Int. Ed. 124, 4408–4411. Shah, S., Sasmal, P.K., Lee, K.B., 2014. J. Mater. Chem. B Mater. Biol. Med. 2, 7685–7693. Shao, Y., Shi, C., Xu, G., Guo, D., Luo, J., 2014. ACS Appl. Mater. Interfaces 6, 10381–10392. Shen, H., Zhou, M., Zhang, Q., Keller, A., Shen, Y., 2015. Colloid Polym. Sci. 293, 1685–1694. Shen, H., Xia, Y., Qin, Z., Wu, J., Zhang, L., Lu, Y., Xia, X., Xu, W., 2017. J. Polym. Sci., Part A: Polym. Chem. 55, 2770–2780. Shibaev, V., Bobrovsky, A., Boiko, N., 2003. Prog. Polym. Sci. 28, 729–836. Shin, D.S., You, J., Rahimian, A., Vu, T., Siltanen, C., Ehsanipour, A., Stybayeva, G., Sutcliffe, J., Revzin, A., 2014. Angew. Chem. Int. Ed. 53, 8221–8224. Shinkai, S., Kinda, H., Manabe, O., 1982. J. Am. Chem. Soc. 104, 2933–2934. Sijbesma, R.P., Beijer, F.H., Brunsveld, L., Folmer, B.J.B., Hirschberg, J.H.K.K., Lange, R.F.M., Lowe, J.K.L., Meijer, E.W., 1997. Science 278, 1601–1604. Siltanen, C., Shin, D.S., Sutcliffe, J., Revzin, A., 2013. Angew. Chem. Int. Ed. 52, 9224–9228. Smet, M., Liao, L.-X., Dehaen, W., McGrath, D.V., 2000. Org. Lett. 2, 511–513. Smith, M.L., Lee, K.M., White, T.J., Vaia, R.A., 2014. Soft Matter 10, 1400–1410. Sogawa, H., Shiotsuki, M., Sanda, F., 2013. Macromolecules 46, 4378–4387. Son, S., Shin, E., Kim, B.S., 2014. Biomacromolecules 15, 628–634. Song, J., Fang, Z., Wang, C., Zhou, J., Duan, B., Pu, L., Duan, H., 2013. Nanoscale 5, 5816–5824. Stellacci, F., Bertarelli, C., Toscano, F., C. Gallazzi, M., Zotti, G., Zerbi, G., 1999. Adv. Mater. 11, 292–295. Stokke, B.T., Draget, K.I., Smidsrod, O., Yuguchi, Y., Urakawa, H., Kajiwara, K., 2000. Macromolecules 33, 1853–1863. Stumpel, J.E., Ziolkowski, B., Florea, L., Diamond, D., Broer, D.J., Schenning, A.P., 2014. ACS Appl. Mater. Interfaces 6, 7268–7274. Sumaru, K., Kikuchi, K., Takagi, T., Yamaguchi, M., Satoh, T., Morishita, K., Kanamori, T., 2013. Biotechnol. Bioeng. 110, 348–352. Sun, R., Xue, C., Ma, X., Gao, M., Tian, H., Li, Q., 2013. J. Am. Chem. Soc. 135, 5990–5993. Sun, L., Zhu, B., Su, Y., Dong, C.-M., 2014. Polym. Chem. 5, 1605–1613. Sun, T., Li, P., Oh, J.K., 2015. Macromol. Rapid Commun. 36, 1742–1748. Sun, Z., Liu, S., Li, K., Tan, L., Cen, L., Fu, G., 2016. Soft Matter 12, 2192–2199. Suzuki, A., Tanaka, T., 1990. Nature 346, 345–347. Swaminathan, S., Garcia-Amoros, J., Fraix, A., Kandoth, N., Sortino, S., Raymo, F.M., 2014. Chem. Soc. Rev. 43, 4167–4178. Szilagyi, A., Sumaru, K., Sugiura, S., Takagi, T., Shinbo, T., Zrinyi, M., Kanamori, T., 2007. Chem. Mater. 19, 2730–2732. Tanaka, F., Mochizuki, T., Liang, X., Asanuma, H., Tanaka, S., Suzuki, K., Kitamura, S.-i., Nishikawa, A., Ui-Tei, K., Hagiya, M., 2010. Nano Lett. 10, 3560–3565. Tanaka, M., Kawai, S., Iwasaki, Y., 2017. J. Biomater. Sci. Polym. Ed. 28, 2021–2033. Tang, R., Liu, Z., Xu, D., Liu, J., Yu, L., Yu, H., 2015. ACS Appl. Mater. Interfaces 7, 8393–8397. Tašič, B., Li, W., Sánchez-Ferrer, A., Čopič, M., Drevenšek-Olenik, I., 2013. Macromol. Chem. Phys. 214, 2744–2751.

REFERENCES 151

ter Schiphorst, J., Coleman, S., Stumpel, J.E., Ben Azouz, A., Diamond, D., Schenning, A.P.H.J., 2015. Chem. Mater. 27, 5925–5931. Theato, P., 2011. Angew. Chem. Int. Ed. 50, 5804–5806. Tibbitt, M.W., Kloxin, A.M., Sawicki, L., Anseth, K.S., 2013. Macromolecules 46. Tokarev, I., Minko, S., 2010. Adv. Mater. 22, 3446–3462. Tomatsu, I., Hashidzume, A., Harada, A., 2005. Macromolecules 38, 5223–5227. Tomatsu, I., Hashidzume, A., Harada, A., 2006. J. Am. Chem. Soc. 128, 2226–2227. Tong, X., Wang, G., Soldera, A., Zhao, Y., 2005. J. Phys. Chem. B 109, 20281–20287. Torras, N., Zinoviev, K.E., Marshall, J.E., Terentjev, E.M., Esteve, J., 2011. Appl. Phys. Lett. 99, 254102/254101–254102/254103. Tsang, K.M., Annabi, N., Ercole, F., Zhou, K., Karst, D., Li, F., Haynes, J.M., Evans, R.A., Thissen, H., Khademhosseini, A., Forsythe, J.S., 2015. Adv. Funct. Mater. 25, 977–986. Tudor, A., Florea, L., Gallagher, S., Burns, J., Diamond, D., 2016. Sensors (Basel) 16, 219. Tyler, D.R., 2003. Coord. Chem. Rev. 246, 291–303. Ube, T., Kawasaki, K., Ikeda, T., 2016. Adv. Mater. 28, 8212–8217. Ube, T., Minagawa, K., Ikeda, T., 2017. Soft Matter 13, 5820–5823. Uchida, K., Takata, A., Ryo, S.-i., Saito, M., Murakami, M., Ishibashi, Y., Miyasaka, H., Irie, M., 2005. J. Mater. Chem. 15, 2128–2133. Ueki, T., Nakamura, Y., Usui, R., Kitazawa, Y., So, S., Lodge, T.P., Watanabe, M., 2015. Angew. Chem. Int. Ed. Eng. 54, 3018–3022. Van Der Veen, G., Prins, W., 1971. Nature 230, 70–72. Ventura, C., Thornton, P., Giordani, S., Heise, A., 2014. Polym. Chem. 5, 6318–6324. Walko, M., Feringa, B.L., 2007. Chem. Commun. Wang, D., Wang, X., 2013. Prog. Polym. Sci. 38, 271–301. Wang, Y., Ma, N., Wang, Z., Zhang, X., 2007. Angew. Chem. Int. Ed. 119, 2881–2884. Wang, X., Yin, J., Wang, X., 2011. Macromolecules 44, 6856–6867. Wang, Y., Lin, S., Zang, M., Xing, Y., He, X., Lin, J., Chen, T., 2012. Soft Matter 8, 3131–3138. Wang, Y., Xu, J.F., Chen, Y.Z., Niu, L.Y., Wu, L.Z., Tung, C.H., Yang, Q.Z., 2014a. Chem. Commun. 50, 7001–7003. Wang, N., Li, Y., Zhang, Y., Liao, Y., Liu, W., 2014b. Langmuir 30, 11823–11832. Wang, X., Yang, Y., Gao, P., Yang, F., Shen, H., Guo, H., Wu, D., 2015a. ACS Macro Lett. 4, 1321–1326. Wang, X., Hu, J., Liu, G., Tian, J., Wang, H., Gong, M., Liu, S., 2015b. J. Am. Chem. Soc. 137, 15262–15275. Wang, M., Zhang, X., Li, L., Wang, J., Wang, J., Ma, J., Yuan, Z., Lincoln, S.F., Guo, X., 2016a. Macromol. Mater. Eng. 301, 191–198. Wang, M., Lin, B., Yang, H., 2016b. Nat. Commun. 7, 13981. Wang, Y., Sun, C.-L., Niu, L.-Y., Wu, L.-Z., Tung, C.-H., Chen, Y.-Z., Yang, Q.-Z., 2017. Polym. Chem. 8, 3596–3602. Watanabe, S., Sato, M., Sakamoto, S., Yamaguchi, K., Iwamura, M., 2000. J. Am. Chem. Soc. 122, 12588–12589. Watanabe, K., Hayasaka, H., Miyashita, T., Ueda, K., Akagi, K., 2015. Adv. Funct. Mater. 25, 2794–2806. Weber, C., Liebig, T., Gensler, M., Pithan, L., Bommel, S., Bléger, D., Rabe, J.P., Hecht, S., Kowarik, S., 2015. Macromolecules 48, 1531–1537. Wei, Y.B., Tang, Q., Gong, C.B., Lam, M.H., 2015. Anal. Chim. Acta 900, 10–20. Willerich, I., Gröhn, F., 2010. Angew. Chem. Int. Ed. 49, 8104–8108. Wondraczek, H., Kotiaho, A., Fardim, P., Heinze, T., 2011. Carbohydr. Polym. 83, 1048–1061. Wu, Y., Zhang, Q., Kanazawa, A., Shiono, T., Ikeda, T., Nagase, Y., 1999. Macromolecules 32, 3951–3956. Xia, D., Yu, G., Li, J., Huang, F., 2014. Chem. Commun. 50, 3606–3608. Xia, D., Wei, P., Shi, B., Huang, F., 2016. Chem. Commun. 52, 513–516.

152

4.  Photoresponsive Polymers

Xiao, W., Zeng, X., Lin, H., Han, K., Jia, H.Z., Zhang, X.Z., 2015. Chem. Commun. 51, 1475–1478. Xing, Q., Li, N., Jiao, Y., Chen, D., Xu, J., Xu, Q., Lu, J., 2015. RSC Adv. 5, 5269–5276. Xu, J.F., Chen, Y.Z., Wu, D., Wu, L.Z., Tung, C.H., Yang, Q.Z., 2013. Angew. Chem. Int. Ed. 52, 9738–9742. Yagai, S., Kitamura, A., 2008. Chem. Soc. Rev. 37, 1520–1529. Yamada, M., Kondo, M., Mamiya, J.-i., Yu, Y., Kinoshita, M., Barrett, C.J., Ikeda, T., 2008. Angew. Chem. Int. Ed. 47, 4986–4988. Yamada, M., Kondo, M., Miyasato, R., Naka, Y., Mamiya, J.-i., Kinoshita, M., Shishido, A., Yu, Y., Barrett, C.J., Ikeda, T., 2009. J. Mater. Chem. 19, 60–62. Yamamoto, S., Tochigi, H., Yamazaki, S., Nakahama, S., Yamaguchi, K., 2016. B. Chem. Soc. Jpn. 89, 481–489. Yan, B., Boyer, J.-C., Branda, N.R., Zhao, Y., 2011. J. Am. Chem. Soc. 133, 19714–19717. Yan, Q., Han, D., Zhao, Y., 2013. Polym. Chem. 4, 5026–5037. Yanagawa, F., Mizutani, T., Sugiura, S., Takagi, T., Sumaru, K., Kanamori, T., 2015a. Colloids Surf. B: Biointerfaces 126, 575–579. Yanagawa, F., Sugiura, S., Takagi, T., Sumaru, K., Camci-Unal, G., Patel, A., Khademhosseini, A., Kanamori, T., 2015b. Adv. Healthc. Mater. 4, 246–254. Yang, J., Ng, M.-K., 2006. Synthesis 3075–3079. Yang, Y., Velmurugan, B., Liu, X., Xing, B., 2013. Small 9, 2937–2944. Yang, J., Li, Z., Zhou, Y., Yu, G., 2014a. Polym. Chem. 5, 6645–6650. Yang, C., Tibbitt, M.W., Basta, L., Anseth, K.S., 2014b. Nat. Mater. 13, 645–652. Yang, H., Tang, R., Wu, W., Liu, W., Guo, Q., Liu, Y., Xu, S., Cao, S., Li, Z., 2016. Polym. Chem. 7, 4016–4024. Yesilyurt, V., Ramireddy, R., Thayumanavan, S., 2011. Angew. Chem. Int. Ed. 50, 3038–3042. Yokoyama, Y., 2000. Chem. Rev. 100, 1717–1739. Yoshino, T., Kondo, M., Mamiya, J.-i., Kinoshita, M., Yu, Y., Ikeda, T., 2010. Adv. Mater. 22, 1361–1363. Yu, H., 2014. J. Mater. Chem. C 2, 3047–3054. Yu, Z., Hecht, S., 2015. J. Polym. Sci., Part A: Polym. Chem. 53, 313–318. Yu, H., Ikeda, T., 2011. Adv. Mater. 23, 2149–2180. Yu, Y., Nakano, M., Ikeda, T., 2003. Nature 425, 145. Yu, H., Iyoda, T., Ikeda, T., 2006. J. Am. Chem. Soc. 11010–11011. Yu, B., Jiang, X., Wang, R., Yin, J., 2010. Macromolecules 43, 10457–10465. Yu, G., Yu, W., Mao, Z., Gao, C., Huang, F., 2015. Small 11, 919–925. Zakhidov, A.A., Yoshino, K., 1995. Synth. Met. 71, 1875–1876. Zhang, W., Yoshida, K., Fujiki, M., Zhu, X., 2011a. Macromolecules 44, 5105–5111. Zhang, X., Wang, C., Lu, X., Zeng, Y., 2011b. J. Appl. Polym. Sci. 120, 3065–3070. Zhang, H., Tian, W., Suo, R., Yue, Y., Fan, X., Yang, Z., Li, H., Zhang, W., Bai, Y., 2015a. J. Mater. Chem. B 3, 8528–8536. Zhang, X., Dong, C., Huang, W., Wang, H., Wang, L., Ding, D., Zhou, H., Long, J., Wang, T., Yang, Z., 2015b. Nanoscale 7, 16666–16670. Zhang, Y., Ang, C.Y., Li, M., Tan, S.Y., Qu, Q., Luo, Z., Zhao, Y., 2015c. ACS Appl. Mater. Interfaces 7, 18179–18187. Zhao, Y., 2007. Chem. Rec. 7, 286–294. Zhao, Y., 2009. J. Mater. Chem. 19, 4887–4895. Zhao, Y., 2012. Macromolecules 45, 3647–3657. Zhao, H., Theato, P., 2013. Polym. Chem. 4, 891–894. Zhao, Y., Tremblay, L., Zhao, Y., 2010. J. Polym. Sci., Part A: Polym. Chem. 48, 4055–4066. Zhao, Y., Tremblay, L., Zhao, Y., 2011a. Macromolecules 44, 4007–4011. Zhao, H., Gu, W., Sterner, E., Russell, T.P., Coughlin, E.B., Theato, P., 2011b. Macromolecules 44, 6433–6440. Zhao, H., Sterner, E.S., Coughlin, E.B., Theato, P., 2012. Macromolecules 45, 1723–1736.

REFERENCES 153

Zhao, X., Qi, M., Liang, S., Tian, K., Zhou, T., Jia, X., Li, J., Liu, P., 2016. ACS Appl. Mater. Interfaces 8, 22127–22134. Zheng, Y.B., Pathem, B.K., Hohman, J.N., Thomas, J.C., Kim, M., Weiss, P.S., 2013. Adv. Mater. 25, 302–312. Zhou, L., Li, J., Luo, Q., Zhu, J., Zou, H., Gao, Y., Wang, L., Xu, J., Dong, Z., Liu, J., 2013. Soft Matter 9, 4635. Zhou, H., Xue, C., Weis, P., Suzuki, Y., Huang, S., Koynov, K., Auernhammer, G.K., Berger, R., Butt, S., Wu, 2017. Nat. Chem. 9, 145–151. Ziółkowski, B., Florea, L., Theobald, J., Benito-Lopez, F., Diamond, D., 2015. J. Mater. Sci. 51, 1392–1399. Zong, C., Zhao, Y., Ji, H., Xie, J., Han, X., Wang, J., Cao, Y., Lu, C., Li, H., Jiang, S., 2016. Macromol. Rapid Commun. 37, 1288–1294.